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{{Short description|Gill-bearing non-tetrapod aquatic vertebrates}}
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{{for-multi|fish as eaten by humans|Fish as food|the superclass of living fish|Osteichthyes|other uses|Fish (disambiguation)}}
{{Use dmy dates|date=October 2017}}
{{Paraphyletic group
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|fossil_range={{fossilrange|535|0}} ] – ]
|image=Balantiocheilos melanopterus - Karlsruhe Zoo 02 (cropped).jpg
|image_caption=], a bony fish
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A '''fish''' ({{plural form}}: '''fish''' or '''fishes''') is an ], ], ]-bearing ] ] with swimming ] and ], but lacking ] with ]. Fish can be grouped into the more ] ] and the more common ], the latter including all ] ] and ], as well as the extinct ]s and ]s. In a break to the long tradition of grouping all fish into a single ] (''Pisces''), contemporary ] views fish as a ] group.


Most fish are ], their body temperature varying with the surrounding water, though some large ] like ] and ] can hold a higher ]. Many fish can ] with each other, such as during ]s. The study of fish is known as ].
'''Fish''' are ] ]s that are typically ectothermic (previously ]), covered with ], and equipped with two sets of paired ]s and several unpaired fins. Fish are abundant in the sea and in fresh water, with species being known from mountain streams (e.g., ] and ]) as well as in the deepest depths of the ocean (e.g., ]s and ]). They are of tremendous importance as ] for people around the world, either collected from the wild (see ]) or farmed in much the same way as ] or ]s (see ]). Fish are also exploited for recreation, through ] and ], and are commonly exhibited in public ]. Fish have an important role in many cultures through the ages, ranging as widely as ] and religious symbols to subjects of books and popular movies.
] at the ], seen swimming among schools of other fish]]
]


The earliest fish appeared during the ] as small ]s; they continued to ] through the ], diversifying into many forms. The earliest fish with dedicated respiratory gills and ], the ]s, had heavy ] that served as protective ]s against ] ]s. The first fish with ]s, the placoderms, appeared in the ] and greatly diversified during the ], the "Age of Fishes".
==Definition==
The term "fish" means to go fuck your mom. Is most precisely used to describe any non-] ], (i.e., an animal with a backbone), that has ]s throughout life and has limbs, if any, in the shape of fins.<ref>{{cite book|title=]|last=Nelson|first=Joseph S.|publisher=], Inc.|year=2006|isbn=0471250317|pages=2}}</ref> Unlike groupings such as ]s or ]s, fish are not a single ] but a ] collection of ], including ]es, ]s, ], ], ], and ].<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0-86542-256-7</ref><ref name="treeoflife">Tree of life web project - .</ref>


Bony fish, distinguished by the presence of ]s and later ] ]s, emerged as the ] group of fish after the ] wiped out the ]s, the placoderms. Bony fish are further divided into the ] and ]. About 96% of all living fish species today are ]s, a ] of ray-finned fish that can ]. The ], a mostly ] ] of vertebrates that have dominated the top ]s in both ] and ]s since the Late ], evolved from lobe-finned fish during the ], developing air-breathing ]s ] to swim bladders. Despite the ] lineage, tetrapods are usually not considered to be fish.
A typical fish is ]; has a ] body that allows it to swim rapidly; extracts oxygen from the water using gills or an accessory breathing organ to enable it to breath atmospheric oxygen; has two sets of paired fins, usually one or two (rarely three) dorsal fins, an anal fin, and a tail fin; has jaws; has skin that is usually covered with ]; and lays eggs that are fertilized internally or externally.


Fish have been an important ] for ]s since ] times, especially ]. ] and ] harvest fish in ] or ] them in ]s or in ]s in the ocean. Fish are caught for ], or raised by ] as ] for private and public exhibition in ] and ]s. Fish have had a role in ] through the ages, serving as ], religious symbols, and as the subjects of art, books and movies.
], a close relative of the ]. Their leaf-like appendages enable them to blend in with floating seaweed]]


== Etymology ==
To each of these there are exceptions. ], ], and some species of ]s show ], and are able to raise their body temperature significantly above that of the ambient water surrounding them.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 83-86, 1997, ISBN 0-86542-256-7</ref> Streamlining and swimming performance varies from highly streamlined and rapid swimmers which are able to reach 10-20 body-lengths per second (such as tuna, salmon, and ]) through to slow but more maneuverable species such as ]s and ] that reach no more than 0.5 body-lengths per second.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 103, 1997, ISBN 0-86542-256-7</ref> Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. ] have paired lungs similar to those of tetrapods, ]s have a structure called the ] that performs a similar function, while many catfish, such as '']'' extract oxygen via the intestine or stomach.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 53-57, 1997, ISBN 0-86542-256-7</ref> Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as ]s, ], ], and ]. Similarly, the surface of the skin may be naked (as in ]s), or covered with scales of a variety of different types usually defined as ] (typical of sharks and rays), ] (fossil lungfishes and coelacanths), ] (various fossil fishes but also living ]s and ]s, ], and ] (these last two are found on most ].<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 33-36, 1997, ISBN 0-86542-256-7</ref> There are even fishes that spend most of their time out of water. ] feed and interact with one another on mudflats and are only underwater when hiding in their burrows.<ref>{{cite web|url=http://64.95.130.5/Summary/SpeciesSummary.php?id=12803|title=Species Summary: ''Periophthalmus barbarus''|author=Froese, R. and D. Pauly. Editors.|publisher=FishBase|accessdate=2006-11-26}}</ref> The ] '']'' lives in underground, ] habitats, and a relative lives in waterlogged ].<ref>{{cite web|url=http://64.95.130.5/Summary/speciesSummary.php?ID=61464|title=Species Summary: ''Phreatobius cisternarum''|author=Froese, R. and D. Pauly. Editors.|publisher=FishBase|accessdate=2006-11-26}}</ref><ref>{{cite web|url=http://www.planetcatfish.com/catelog/species.php?species_id=646|title=Cat-eLog: Heptapteridae: ''Phreatobius'': ''Phreatobius'' sp. (1)|author=Planet Catfish|publisher=Planet Catfish|accessdate=2006-11-26}}</ref>


The word ''fish'' is inherited from ], and is related to ] {{lang|de|Fisch}}, the ] {{lang|la|piscis}} and ] {{lang|sga|īasc}}, though the exact root is unknown; some authorities reconstruct a ] root {{lang|ine-x-proto|*peysk-}}, attested only in ], ], and ].<ref>{{Cite web |title=DWDS – Digitales Wörterbuch der deutschen Sprache |url=https://www.dwds.de/wb/Fisch |access-date=2023-01-21 |website=DWDS |language=de |archive-date=31 July 2020 |archive-url=https://web.archive.org/web/20200731172104/https://www.dwds.de/wb/Fisch |url-status=live}}</ref><ref>Winfred Philipp Lehmann, Helen-Jo J. Hewitt, Sigmund Feist, ''A Gothic etymological dictionary'', 1986, ''s.v.'' ''fisks'' p. 118</ref><ref>{{cite web |title=fish, n.1 |url=https://www.oed.com/view/Entry/70646 |publisher=] |access-date=2023-01-21 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010607/https://www.oed.com/view/Entry/70646 |url-status=live}}</ref><ref>{{cite book |last=Buck |first=Carl Darling |author-link=Carl Darling Buck |title=A Dictionary of Selected Synonyms in the Principal Indo-European Languages |year=1949 |chapter=section 3.65 |page=184}}</ref>
Fish range in size from the 16&nbsp;m (51&nbsp;ft) ] to the 8&nbsp;mm (just over ¼ of an inch) long ].


== Evolution ==
Many types of ]s commonly referred to as "fish" are not fish in the sense given above; examples include ], ], ], ] and ]. In earlier times, even biologists did not make a distinction - sixteenth century natural historians classified also ]s, ]s, ]s, ]s, even ]es, as well as a host of aquatic invertebrates, as fish<ref>Jr.Cleveland P Hickman, Larry S. Roberts, Allan L. Larson: Integrated Principles of Zoology, McGraw-Hill Publishing Co, 2001, ISBN 0–07–290961–7</ref>. In some contexts, especially in aquaculture, the true fish are referred to '''finfish''' to distinguish them from these other animals.


{{main|Evolution of fish}}
==Classification==
Fish are a ] group: that is, any ] containing all fish also contains the ]s, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications.
<div style="-moz-column-count:2; column-count:2;">
Fish are classified into the following major groups:
* Subclass ] (early jawless fish)
* Class ]
* Class ]
* (unranked) ] (early jawless fish)
** (unranked) ]
*** ] (]s)
** Class ]
** Class ]
** Class ]
* Infraphylum ] (jawed vertebrates)
** Class ] (armoured fishes, extinct)
** Class ] (cartilaginous fish)
** Class ] (spiny sharks, extinct)
** Superclass ] (bony fish)
*** Class ] (ray-finned fish)
*** Class ] (lobe-finned fish)
**** Subclass ] (]s)
**** Subclass ] (])
</div>
Some palaeontologists consider that ] are ]s, and so regard them as primitive fish. For a fuller treatment of classification, see the ] article.


=== Fossil history ===
The various fish groups taken together account for more than half of the known vertebrates. There are almost 28,000 known ] species of fish, of which almost 27,000 are bony fish, with the remainder being about 970 ] and about 108 hagfishes and lampreys.<ref>Nelson, J. S.: Fishes of the World, John Wiley & Sons, Inc., p 4-5, 2006 ISBN 0471250317</ref> A third of all of these species are contained within the nine largest families; from largest to smallest, these families are ], ], ], ], ], ], ], ], and ]. On the other hand, about 64 families are ], containing only one species. It is predicted that the eventual number of total extant species will be at least 32,500.<ref>Nelson, J. S.: Fishes of the World, John Wiley & Sons, Inc., p 3, 2006 ISBN 0471250317</ref>
</font color>


{{further|Timeline of fish evolution}}
==Anatomy==
{{main|Fish anatomy}}
[[Image:Lampanyctodes hectoris (Hector's lanternfish)2.png|thumb|left|350px|The anatomy of ''Lampanyctodes hectoris'' <br /><small>
(1) - operculum (gill cover), (2) - lateral line, (3) - dorsal fin, (4) - fat fin, (5) - caudal peduncle, (6) - caudal fin, (7) - anal fin, (8) - photophores, (9) - pelvic fins (paired), (10) - pectoral fins (paired)</small>]]


]'' was a giant ] armoured ], c. 400 ].]]
=== Digestive system ===
The advent of jaws allowed fish to eat a much wider variety of food, including plants and other organisms. In fish, food is ingested through the mouth and then broken down in the ]. When it enters the stomach, the food is further broken down and, in many fish, further processed in fingerlike pouches called ]. The pyloric caeca secrete digestive ] and absorb nutrients from the digested food. Organs such as the ] and ] add enzymes and various digestive chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption


About 530 million years ago during the ], fishlike animals with a ] and eyes at the front of the body, such as '']'', appear in the ].<ref name="Shu 2003">{{cite journal |last1=Shu |first1=D. G. |title=Head and backbone of the Early Cambrian vertebrate ''Haikouichthys'' |journal=Nature |first2=S. |last2=Conway Morris |first3=J. |last3=Han |first4=Z. F. |last4=Zhang |first5=K. |last5=Yasui |first6=P. |last6=Janvier |first7=L. |last7=Chen |first8=X. L. |last8=Zhang |first9=J. N. |last9=Liu |last10=Li |first10=Y. |last11=Liu |first11=H.-Q. |display-authors=6 |volume=421 |issue=6922 |doi=10.1038/nature01264 |bibcode=2003Natur.421..526S |pmid=12556891 |url=https://www.researchgate.net/publication/10926399 |pages=526–529 |date =2003 |s2cid=4401274 }}</ref> During the late ], other jawless forms such as ]s appear.<ref name="Donoghue Purnell 2009">{{cite journal |last1=Donoghue |first1=Philip C. J. |last2=Purnell |first2=Mark A. |title=The Evolutionary Emergence of Vertebrates From Among Their Spineless Relatives |journal=Evolution: Education and Outreach |volume=2 |issue=2 |date=2009 |issn=1936-6426 |doi=10.1007/s12052-009-0134-3 |pages=204–212 |doi-access=free }}</ref><ref name="Miller Clark 1984">{{cite journal |last1=Miller |first1=James F. |last2=Clark |first2=D. L. |title=Cambrian and earliest Ordovician conodont evolution, biofacies, and provincialism |journal=Geological Society of America Special Paper |series=Geological Society of America Special Papers |issue=196 |year=1984 |volume=196 |pages=43–68 |doi=10.1130/SPE196-p43 |isbn=978-0-8137-2196-5 |url=https://books.google.com/books?id=bGGi7wlAuw4C&pg=PA43}}</ref>
=== Respiratory system ===
Most fish exchange gases by using ]s that are located on either side of the ]. Gills are made up of threadlike structures called ]s. Each filament contains a network of ] that allow a large ] for the exchange of ] and ]. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gill filaments. The blood in the capillaries flows in the opposite direction to the water, causing ]. They then push the oxygen-poor water out through openings in the sides of the pharynx. Some fishes, like ]s and ]s, possess multiple gill openings. However, most fishes have a single gill opening on each side of the body. This opening is hidden beneath a protective bony cover called an ].


] appear in the ], with giant armoured ] such as '']''.<ref>{{cite news |url=http://www.smh.com.au/news/science/jaws-of-steel-on-this-fish-tank/2006/11/29/1164777657728.html |title=Monster fish crushed opposition with strongest bite ever |newspaper=] |date=30 November 2006 |access-date=26 February 2013 |url-status=live |archive-url=https://web.archive.org/web/20130402143409/http://www.smh.com.au/news/science/jaws-of-steel-on-this-fish-tank/2006/11/29/1164777657728.html |archive-date=2 April 2013}}</ref> Jawed fish, too, appeared during the Silurian:<ref name="Choo Min Wenjin 2014">{{cite journal |last1=Choo |first1=Brian |last2=Zhu |first2=Min |last3=Zhao |first3=Wenjin |last4=Jia |first4=Liaotao |last5=Zhu |first5=You'an |title=The largest Silurian vertebrate and its palaeoecological implications |journal=Scientific Reports |date=2014 |volume=4 |pages=5242 |doi=10.1038/srep05242 |pmid=24921626 |pmc=4054400 |bibcode=2014NatSR...4.5242C}}</ref> the cartilaginous ]<ref name="Andreev Sansom 2022a">{{cite journal |last1=Andreev |first1=Plamen S. |last2=Sansom |first2=Ivan J. |last3=Li |first3=Qiang |last4=Zhao |first4=Wenjin |last5=Wang |first5=Jianhua |last6=Wang |first6=Chun-Chieh |last7=Peng |first7=Lijian |last8=Jia |first8=Liantao |last9=Qiao |first9=Tuo |last10=Zhu |first10=Min |display-authors=5 |date=September 2022 |title=Spiny chondrichthyan from the lower Silurian of South China |url=https://www.nature.com/articles/s41586-022-05233-8 |journal=Nature |volume=609 |issue=7929 |pages=969–974 |doi=10.1038/s41586-022-05233-8 |pmid=36171377 |bibcode=2022Natur.609..969A |s2cid=252570103}}</ref><ref name="Andreev Sansom 2022b">{{Cite journal |last1=Andreev |first1=Plamen S. |last2=Sansom |first2=Ivan J. |last3=Li |first3=Qiang |last4=Zhao |first4=Wenjin |last5=Wang |first5=Jianhua |last6=Wang |first6=Chun-Chieh |last7=Peng |first7=Lijian |last8=Jia |first8=Liantao |last9=Qiao |first9=Tuo |last10=Zhu |first10=Min |display-authors=6 |date=September 2022 |title=The oldest gnathostome teeth |url=http://dx.doi.org/10.1038/s41586-022-05166-2 |journal=Nature |volume=609 |issue=7929 |pages=964–968 |bibcode=2022Natur.609..964A |doi=10.1038/s41586-022-05166-2 |issn=0028-0836 |pmid=36171375 |s2cid=252569771}}</ref> and the bony ].<ref>{{cite book |last1=Berg |first1=Linda R.|last2=Solomon |first2=Eldra Pearl |last3=Martin |first3=Diana W. |title=Biology |year=2004 |publisher=] |isbn=978-0-534-49276-2 |page=599}}</ref>
Juvenile ]s have external gills, a very primitive feature that they hold in common with larval ]s.


During the ], fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes".{{sfn|Benton|2005|loc=p. 35: Fig 2.10, p. 73: Fig 3.25}}<ref name="Dalton 2006">{{cite journal |last=Dalton |first=Rex |title=Hooked on fossils |journal=Nature |date=January 2006 |volume=439 |issue=7074 |pages=262–263 |doi=10.1038/439262a |pmid=16421540 |s2cid=4357313 |doi-access=free }}</ref>
]
Many fish can breathe air. The mechanisms for doing so are varied. The skin of anguillid eels may be used to absorb oxygen. The buccal cavity of the ] may be used to breathe air. Catfishes of the families ], ], and ] are able to absorb air through their digestive tracts.<ref name="Armbruster1998">{{cite journal|url=http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf|title=Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes|journal=]|year=1998|issue=3|pages=663-675|format=]}}</ref> ] and ]s have paired lungs similar to those of ]s and must rise to the surface of the water to gulp fresh air in through the mouth and pass spent air out through the gills. ] and ] have a vascularised swim bladder that is used in the same way. ]es, ], and many ] breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to what frogs do). A number of fishes have evolved so-called '''accessory breathing organs''' that are used to extract oxygen from the air. Labyrinth fish (such as ]s and ]s) have a ] above the gills that performs this function. A few other fish have structures more or less resembling labyrinth organs in form and function, most notably ], ], and the ] family of catfish.


=== Phylogeny ===
Being able to breathe air is primarily of use to fish that inhabit shallow, seasonally variable waters where the oxygen concentration in the water may decline at certain times of the year. At such times, fishes dependent solely on the oxygen in the water, such as perch and cichlids, will quickly suffocate, but air-breathing fish can survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some of these air-breathing fish are able to survive in damp burrows for weeks after the water has otherwise completely dried up, entering a state of ] until the water returns.


Fishes are a ] group, since any ] containing all fish, such as the ] or (for bony fish) ], also contains the clade of ]s (four-limbed vertebrates, mostly terrestrial), which are usually not considered fish.<ref>{{Cite journal |last=Greene |first=Harry W. |date=1998-01-01 |title=We are primates and we are fish: Teaching monophyletic organismal biology |journal=Integrative Biology |volume=1 |issue=3 |pages=108–111 |doi=10.1002/(sici)1520-6602(1998)1:3<108::aid-inbi5>3.0.co;2-t |issn=1520-6602}}</ref><ref name="Nelson 2016"/> Some tetrapods, such as ]ns and ], have ] a fish-like body shape through ].<ref>{{cite book |last=Davis |first=R. W. |year=2019 |chapter=Return to the Sea: The Evolution of Marine Mammals |pages=7–27 |editor-last=Davis |editor-first=R. W. |title=Marine Mammals: Adaptations for an Aquatic Life |publisher=] |location=New York |isbn=978-3-3199-8278-6}}</ref> '']'' comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods".<ref name="Nelson 2016"/> The ] of extant fish is unevenly distributed among the various groups; ], bony fishes ], make up 96% of fish species.{{sfn|Benton|2005|pp=175–184}}<ref name="Nelson 2016"/> The ]<ref name="Friedman & Sallan 2012">{{cite journal |last1=Friedman |first1=Matt |last2=Sallan |first2=Lauren Cole |title=Five hundred million years of extinction and recovery: A Phanerozoic survey of large-scale diversity patterns in fishes |journal=Palaeontology |date=June 2012 |volume=55 |issue=4 |pages=707–742 |doi=10.1111/j.1475-4983.2012.01165.x |bibcode=2012Palgy..55..707F |s2cid=59423401 |doi-access=free }}</ref> shows the ] of all groups of living fishes (with their respective diversity<ref name="Nelson 2016"/>) and the tetrapods.<ref name=IUCN2023>{{cite web |website=] |version=2023.1 |title=Summary Statistics |url=https://www.iucnredlist.org/resources/summary-statistics |access-date=5 February 2024}} Table 1a: <!--counts 38,595 for tetrapods, using ASW6, RD, Birdlife and MDD--></ref> ] groups are marked with a ] (†); groups of uncertain placement<ref name="Friedman & Sallan 2012"/> are labelled with a question mark (?) and dashed lines (- - - - -).
]
Fish can be divided into '''obligate air breathers''' and '''facultative air breathers'''. Obligate air breathers, such as the African lungfish, ''must'' breathe air periodically or they will suffocate. Facultative air breathers, such as the catfish ''Hypostomus plecostomus'', will only breathe air if they need to and will otherwise rely solely on their gills for oxygen if conditions are favourable. Most air breathing fish are not obligate air breathers, as there is an energetic cost in rising to the surface and a fitness cost of being exposed to surface predators.<ref name="Armbruster1998"/>


{{clade| style=font-size:100%;line-height:100%;
=== Circulatory system ===
|label1=]
Fish have a ] with a ] that pumps the ] in a single loop throughout the body. The blood goes from the heart to gills, from the gills to the rest of the body, and then back to the heart. In most fish, the heart consists of four parts: the ], the ], the ], and the ]. Despite consisting of four parts, the fish heart is still a two-chambered heart.<ref name="Setaro>{{Citation | last =Setaro | first =John F. | year =1999 | title =Circulatory System | publisher =Microsoft Encarta 99}}</ref> The sinus venosus is a thin-walled sac that collects blood from the fish's ]s before allowing it to flow to the atrium, which is a large muscular chamber. The atrium serves as a one-way compartment for blood to flow into the ventricle. The ventricle is a thick-walled, muscular chamber and it does the actual pumping for the heart. It pumps blood to a large tube called the bulbus arteriosus. At the front end, the bulbus arteriosus connects to a large blood vessel called the aorta, through which blood flows to the fish's gills.
|1={{clade
|1=] (118 species: ], ]) ]
|2={{clade
|label1= ? <!--uncertain placement-->|state=dashed
|1= †], †], †] ] ] ]
|2={{clade
|1=†] ]
|2={{clade
|1=†] ]
|label2=]|sublabel2=]
|2={{clade
|1=†] ]
|2={{clade
|label1 = ? <!--uncertain placement-->|state=dashed
|1=†] ]
|2={{clade
|label1=]
|1=&nbsp;(>1,100 species: ], ], ]) ]
|label2=]<!--bony fishes INC tetrapods-->
|2={{clade
|1={{clade
|label1=]
|1={{clade
|label1=] |1=&nbsp;(2 species: ]s) ]
|label2=]
|2={{clade
|1=] (6 species: ]) ]
|2=] (>38,000 species, not considered fish: amphibians, reptiles, birds, mammals) ]
}}
}}
|label2=]
|2={{clade
|label1=] |1=&nbsp;(14 species: ], ]) ]
|label2=]
|2={{clade
|label1=] |1=&nbsp;(27 species: ], ]) ]
|label2=]
|2={{clade
|1={{clade
|label1=]
|1={{clade
|1=] (7 species: ]s, ]s) ]
|2=] (2 species: ], ]) ]
}}
|label2=]
|2=&nbsp;(>32,000 species) ]
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}


=== Excretory system=== === Taxonomy ===
As with many aquatic animals, most fish release their nitrogenous wastes as ]. Some of the wastes ] through the gills into the surrounding water. Others are removed by the ]s, excretory organs that ] wastes from the blood. Kidneys help fishes control the amount of ammonia in their bodies. Saltwater fish tend to lose water because of ]. In saltwater fish, the kidneys concentrate wastes and return as much water as possible back to the body. The reverse happens in ], they tend to gain water continuously. The kidneys of freshwater fish are specially adapted to pump out large amounts of dilute urine. Some fish have specially adapted kidneys that change their function, allowing them to move from freshwater to saltwater.


{{main|Taxonomy of fish}}
=== Sensory and nervous system ===
].]]
====Central nervous system====
Fish typically have quite small ]s relative to body size when compared with other vertebrates, typically one-fifteenth the mass of the brain from a similarly sized bird or mammal.<ref name="Helfman_brain">Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 48-49, 1997, ISBN 0-86542-256-7</ref> However, some fish have relatively large brains, most notably ] and ]s, which have brains of about as massive relative to body weight as ]s and ]s.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 191, 1997, ISBN 0-86542-256-7</ref>


Fishes (without tetrapods) are a ] group and for this reason, the class ''Pisces'' seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three ] ] (Agnatha, Chondrichthyes, and Osteichthyes), and with extinct forms sometimes classified within those groups, sometimes as their own classes.<ref>{{cite book |last=Benton |first=M.J. |year=1998 |chapter=The quality of the fossil record of vertebrates |pages= 269–303, Fig. 2 |editor1=Donovan, S.K. |editor2=Paul, C.R.C. |title=The adequacy of the fossil record |publisher=]}}</ref>
The brain is divided into several regions. At the front are the ], a pair of structure the receive and process signals from the ]s via the two ]s.<ref name="Helfman_brain"/> The olfactory lobes are very large in fishes that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed ], the equivalent structure to the ] in higher vertebrates. In fishes the telencephalon is concerned mostly with ].<ref name="Helfman_brain"/> Together these structures form the '''forebrain'''.


Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within the nine largest families; from largest to smallest, these are ], ], ], ], ], ], ], ], and ]. About 64 families are ], containing only one species.<ref name="Nelson 2016">{{harvnb|Nelson|2016|p=3}}</ref>
Connecting the forebrain to the ''midbrain'' is the ] (in the adjacent diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs a number of functions associated with ]s and ].<ref name="Helfman_brain"/> The ] lies just above the diencephalon. This structure performs many different functions including detecting light, maintaining ] rhythms, and controlling colour changes.<ref name="Helfman_brain"/>


=== Diversity ===
The ] or mesencephalon contains the two ]s. These are very large in species that hunt by sight, such as ] and ]s.<ref name="Helfman_brain"/>


{{main|Diversity of fish}}
The '''hindbrain''' or ] is particularly involved in swimming and balance.<ref name="Helfman_brain"/> The cerebellum is a single-lobed structure that is usually very large, typically the biggest part of the brain.<ref name="Helfman_brain"/> Hagfish and ]s have relatively small cerebellums, but at the other extreme the cerebellums of ]s are massively developed and apparently involved in their ].<ref name="Helfman_brain"/>


Fish range in size from the huge {{convert|16|m|adj=on}} ]<ref name="McClain Balk Benfield Branch 2015">{{cite journal |last1=McClain |first1=Craig R. |last2=Balk |first2=Meghan A. |last3=Benfield |first3=Mark C. |last4=Branch |first4=Trevor A. |last5=Chen |first5=Catherine |last6=Cosgrove |first6=James |last7=Dove |first7=Alistair D.M. |last8=Gaskins |first8=Leo |last9=Helm |first9=Rebecca R. |last10=Hochberg |first10=Frederick G. |last11=Lee |first11=Frank B. |last12=Marshall |first12=Andrea |last13=McMurray |first13=Steven E. |last14=Schanche |first14=Caroline |last15=Stone |first15=Shane N. |last16=Thaler |first16=Andrew D. |display-authors=5 |title=Sizing ocean giants: patterns of intraspecific size variation in marine megafauna |journal=] |volume=3 |date=2015-01-13 |issn=2167-8359 |doi=10.7717/peerj.715 |doi-access=free |page=e715 |pmid=25649000 |pmc=4304853 }}</ref> to some tiny teleosts only {{convert|8|mm|sigfig=1|adj=on}} long, such as the cyprinid '']''<ref name="Kottelat">{{cite journal |last1=Kottelat |first1=Maurice |last2=Britz |first2=Ralf |last3=Heok Hui|first3=Tan |last4=Witte |first4=Kai-Erik |year=2005 |title=''Paedocypris'', a new genus of Southeast Asian cyprinid fish with a remarkable sexual dimorphism, comprises the world's smallest vertebrate |journal=Proceedings of the Royal Society B |volume=273 |issue=1589 |pages=895–899 |doi=10.1098/rspb.2005.3419 |url=http://www.eurocean.org/np4/file/133/Paedocypris_20__20the_20world_20s_20smal.pdf |accessdate=26 October 2012 |pmid=16627273 |pmc=1560243 |url-status=dead |archive-url=https://web.archive.org/web/20090712211749/http://www.eurocean.org/np4/file/133/Paedocypris_20__20the_20world_20s_20smal.pdf|archive-date=12 July 2009}}</ref> and the ].<ref name=fishbase>{{FishBase|genus=Schindleria|species=brevipinguis |year=2017 |month=September}}</ref>
The '''brain stem''' or ] is the most posterior part of the brain.<ref name="Helfman_brain"/> As well as controlling the functions of some of the muscles and body organs, in bony fish at least the brain stem is also concerned with ] and ].<ref name="Helfman_brain"/>


<gallery mode="packed" heights=175>
====Sense organs====
File:Rhincodon typus fgbnms (cropped).jpg|Largest: ]
Most fish possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least as good as a human's. Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears in their heads, many fish may not hear sounds very well. However, most fishes have sensitive receptors that form the ]. The lateral line system allows for many fish to detect gentle currents and vibrations, as well as to sense the motion of other nearby fish and prey.<ref name="Encarta 99">{{Citation | last =Orr | first =James | year =1999 | title =Fish | publisher =Microsoft Encarta 99}}</ref> Some fishes such as catfishes and sharks, have organs that detect low levels electric current.<ref>Albert, J.S., and W.G.R. Crampton. 2005. Electroreception and electrogenesis. Pp. 431-472 in The Physiology of Fishes, 3rd Edition. D.H. Evans and J.B. Claiborne (eds.). CRC Press.</ref> Other fish, like the electric eel, can produce their own electricity.
File:Paedocypris progenetica 001.jpg|Smallest: e.g. '']''
</gallery>


Swimming performance varies from fish such as tuna, ], and ] that can cover 10–20 body-lengths per second to species such as ]s and ] that swim no more than 0.5 body-lengths per second.{{sfn|Helfman|Collette|Facey|1997|p=103}}
====Pain reception in fish====
In 2003, Scottish ]s at the ] performing research on rainbow trout concluded that fish exhibit behaviors often associated with ].<ref>{{cite web|url=http://news.bbc.co.uk/1/hi/sci/tech/2983045.stm|title=Fish do feel pain, scientists say}}</ref> Professor James D. Rose of the ] critiqued the study, claiming it was flawed.<ref></</ref> Rose had published his own study a year earlier arguing that fish cannot feel pain as they lack the appropriate ] in the brain.<ref>James D. Rose, , 2002. Retrieved September 27, 2007.</ref>


<gallery mode="packed" heights=100>
===Muscular system===
File:Salmo salar.jpg|Fastest: e.g. ], 10–20 body lengths/second
{{main|Fish locomotion}}
File:Anguilla japonica 1856.jpg|Slowest: e.g. ], 0.5 body lengths/second
Most fish move by contracting paired sets of muscles on either side of the backbone alternately. These contractions form S-shaped curves that move down the body of the fish. As each curve reaches the back fin, backward force is created. This backward force, in conjunction with the fins, moves the fish forward. The fish's fins are used like an airplane's stabilizers. Fins also increase the surface area of the tail, allowing for an extra boost in speed. The streamlined body of the fish decreases the amount of friction as they move through water. Since body tissue is more dense than water, fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a ] that adjust their buoyancy through manipulation of gases.
</gallery>


A typical fish is ], has a ] body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and a tail fin, jaws, skin covered with ], and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes.{{sfn|Helfman|Collette|Facey|1997|pp=3, 33–36}}
=== Reproductive system ===
{{See|Spawning}}
====Organs====
]: 1. ], 2. ], 3. ], 4. ], 5. ], 6. ]]]
Fish reproductive organs include ] and ]. In most fish species, gonads are paired organs of similar size, which can be partially or totally fused.<ref name="Guimaraes"/> There may also be a range of secondary reproductive organs that help in increasing a fish's fitness.


<gallery mode="packed" heights=120>
In terms of ] distribution, the structure of teleosts testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorph fishes they are confined to the distal portion of these structures. Fishes can present cystic or semi-cystic ] in relation to the phase of release of germ cells in cysts to the seminiferous tubules lumen.<ref name="Guimaraes">{{cite journal|last=Guimaraes-Cruz, Rodrigo J.|coauthors=Santos, José E. dos; Santos, Gilmar B.|title=Gonadal structure and gametogenesis of ''Loricaria lentiginosa'' Isbrücker (Pisces, Teleostei, Siluriformes)|journal=Rev. Bras. Zool.|date=July/Sept. 2005|volume=22|issue=3|pages=556-564|id=ISSN 0101-8175}}</ref>
File:Humpback anglerfish.png|]:<br />]
File:Atl mackerel photo3 exp.jpg|], somewhat ]:<br />]
File:Hippocampus hippocampus (cropped).jpg|Tail not ]:<br />]
File:Phycodurus eques P2023146 (cropped).JPG|]d:<br />]
File:Eastern Cleaner Clingfish (cropped).jpg|No ]:<br />]
File:Cyphotilapia frontosa mouthbrooding.jpg|]: ] with ] in mouth
</gallery>


== Ecology ==
Fish ovaries may be of three types: ], secondary gymnovarian or ]. In the first type, the oocytes are released directly into the ]ic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the ] and then they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the ].<ref name="Brito">{{cite journal|url=http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0102-09352003000500018|title=Reproduction of the surubim catfish (Pisces, Pimelodidae) in the São Francisco River, Pirapora Region, Minas Gerais, Brazil|last=Brito|first=M.F.G.|coauthors=Bazzoli, N.|journal=Arquivo Brasileiro de Medicina Veterinária e Zootecnia|volume=55|issue=5|year=2003|id=ISSN: 0102-0935}}</ref> Gymnovaries are the primitive condition found in lungfishes, sturgeons, and bowfins. Cystovaries are the condition that characterizes most of the teleosts, where the ovary lumen has continuity with the oviduct.<ref name="Guimaraes"/> Secondary gymnovaries are found in salmonids and a few other teleosts.
=== Habitats ===


]
Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilization processes. Changes in the nucleus, ooplasm, and the surrounding layers characterize the oocyte maturation process.<ref name="Guimaraes"/>


Fish species are roughly divided equally between ] and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species.<ref name="Manel Guerin Mouillot Blanchet 2020">{{cite journal |last1=Manel |first1=Stéphanie |last2=Guerin |first2=Pierre-Edouard |last3=Mouillot |first3=David |last4=Blanchet |first4=Simon |last5=Velez |first5=Laure |last6=Albouy |first6=Camille |last7=Pellissier |first7=Loïc |title=Global determinants of freshwater and marine fish genetic diversity |journal=Nature Communications |volume=11 |issue=1 |date=2020-02-10 |page=692 |issn=2041-1723 |pmid=32041961 |pmc=7010757 |doi=10.1038/s41467-020-14409-7|bibcode=2020NatCo..11..692M }}</ref> ]s in the ] constitute the center of diversity for marine fishes,<ref name="Hubert Meyer Bruggemann Guérin 2012">{{cite journal |last1=Hubert |first1=Nicolas |last2=Meyer |first2=Christopher P. |last3=Bruggemann |first3=Henrich J. |last4=Guérin |first4=Fabien |last5=Komeno |first5=Roberto J. L. |last6=Espiau |first6=Benoit |last7=Causse |first7=Romain |last8=Williams |first8=Jeffrey T. |last9=Planes |first9=Serge |display-authors=5 |title=Cryptic Diversity in Indo-Pacific Coral-Reef Fishes Revealed by DNA-Barcoding Provides New Support to the Centre-of-Overlap Hypothesis |journal=PLOS ONE |volume=7 |issue=3 |date=2012-03-15 |pmid=22438862 |pmc=3305298 |doi=10.1371/journal.pone.0028987 |doi-access=free |page=e28987 |bibcode=2012PLoSO...728987H }}</ref> whereas continental freshwater fishes are most diverse in large ]s of ]s, especially the ], ], and ] basins.<ref name="van der Sleen Albert 2022">{{cite book |last1=van der Sleen |first1=Peter |last2=Albert |first2=James S. |title=Encyclopedia of Inland Waters |chapter=Patterns in Freshwater Fish Diversity |publisher=Elsevier |date=2022 |isbn=978-0-12-822041-2 |doi=10.1016/b978-0-12-819166-8.00056-6 |pages=243–255}}</ref> More than 5,600 fish species inhabit ]al freshwaters alone, such that ]es represent about 10% of all ] species on the Earth.<ref name="Albert Carvalho Petry 2011">{{cite journal |last1=Albert |first1=James S. |last2=Carvalho |first2=Tiago P. |last3=Petry |first3=Paulo |title=Aquatic Biodiversity in the Amazon: Habitat Specialization and Geographic Isolation Promote Species Richness |journal=Animals |date=June 2011 |volume=1 |issue=2 |pages=205–241 |doi=10.3390/ani1020205 |doi-access=free |pmid=26486313|pmc=4513461 }}</ref>
Postovulatory follicles are structures formed after oocyte release; they do not have ] function, present a wide irregular lumen, and are rapidly reabosrbed in a process involving the apoptosis of follicular cells. A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages.<ref name="Guimaraes"/>


Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high ]s (e.g., ] and ]) to the ] and even ] depths of the deepest oceans (e.g., ] and ]), although none have been found in the deepest 25% of the ocean.<ref>{{cite journal|last1=Yancey |first1=P.H. |last2=Gerringer |first2=M.E. |last3=Drazen |first3=J.C. |last4=Rowden |first4=A.A. |last5=Jamieson |first5=A. |title=Marine fish may be biochemically constrained from inhabiting the deepest ocean depths |year=2014 |journal=PNAS |pmc=3970477 |pmid=24591588 |doi=10.1073/pnas.1322003111 |volume=111 |issue=12 |pages=4461–4465 |bibcode=2014PNAS..111.4461Y |doi-access=free }}</ref> The deepest living fish in the ocean so far found is a cusk-eel, '']'', recorded at the bottom of the ] at {{convert|8370|m|ft|abbr=on}}.<!--<ref name="EoF">{{cite book |editor1=Paxton, J.R. |editor2=Eschmeyer, W.N. |last=Nielsen |first=Jørgen G. |year=1998 |title=Encyclopedia of Fishes |publisher=] |location=San Diego |page=134 |isbn=0-12-547665-5}}</ref>--><ref>{{cite web |title=What is the deepest-living fish? |url=http://australianmuseum.net.au/what-is-the-deepest-living-fish |work=] |date=23 December 2014 |access-date=18 September 2015 }}</ref>
Some fish are ]s, having testes and ovaries either at different phases in their life cycle or, like ], can be simultaneously male and female.


In terms of temperature, ] live in cold{{efn|The temperature is often around 0 C. The freezing point of seawater at the surface is -1.85 C, falling to -2.62 C at a depth of 1000 metres. However, the water can be ] somewhat below these temperatures.<ref name="Haumann Moorman Riser Smedsrud 2020">{{cite journal |last1=Haumann |first1=F. Alexander |last2=Moorman |first2=Ruth |last3=Riser |first3=Stephen C. |last4=Smedsrud |first4=Lars H. |last5=Maksym |first5=Ted |last6=Wong |first6=Annie P.S. |last7=Wilson |first7=Earle A. |last8=Drucker |first8=Robert |last9=Talley |first9=Lynne D. |last10=Johnson |first10=Kenneth S. |last11=Key |first11=Robert M. |last12=Sarmiento |first12=Jorge L. |display-authors=5 |title=Supercooled Southern Ocean Waters |journal=Geophysical Research Letters |volume=47 |issue=20 |date=2020-10-28 |doi=10.1029/2020GL090242 |bibcode=2020GeoRL..4790242H |hdl=1912/26495 |hdl-access=free }}</ref>}} waters of the Southern Ocean, including under the ] at a latitude of 79°S,<ref name="Purser Hehemann 2022">{{Cite journal |last1=Purser |first1=Autun |last2=Hehemann |first2=Laura |last3=Boehringer |first3=Lilian |last4=Tippenhauer |first4=Sandra |last5=Wege |first5=Mia |last6=Bornemann |first6=Horst |last7=Pineda-Metz |first7=Santiago E. A. |last8=Flintrop |first8=Clara M. |last9=Koch |first9=Florian |last10=Hellmer |first10=Hartmut H. |last11=Burkhardt-Holm |first11=Patricia |last12=Janout |first12=Markus |last13=Werner |first13=Ellen |last14=Glemser |first14=Barbara |last15=Balaguer |first15=Jenna |display-authors=5 |date=2022 |title=A vast icefish breeding colony discovered in the Antarctic |journal=Current Biology |volume=32 |issue=4 |pages=842–850.e4 |doi=10.1016/j.cub.2021.12.022 |pmid=35030328 |s2cid=245936769 |doi-access=free |bibcode=2022CBio...32E.842P |hdl=2263/90796 |hdl-access=free}}</ref> while ] live in desert springs, streams, and marshes, sometimes highly saline, with water temperatures as high as 36 C.<ref name="USFWS 1993">{{cite web |publisher=] |year=1993 |title=Desert Pupfish (Cyprinodon macularius) Recovery Plan |first1=Paul C. |last1=Marsh |first2=Donald W |last2=Sada |archive-url=https://web.archive.org/web/20111017200006/https://ecos.fws.gov/docs/recovery_plan/931208b.pdf |archive-date=2011-10-17 |url=https://ecos.fws.gov/docs/recovery_plan/931208b.pdf |url-status=live}}</ref><ref name="Shrode Gerking 1977">{{Cite journal |last1=Shrode |first1=Joy B. |last2=Gerking |first2=Shelby D. |year=1977 |title=Effects of Constant and Fluctuating Temperatures on Reproductive Performance of a Desert Pupfish, Cyprinodon n. nevadensis |url=http://dx.doi.org/10.1086/physzool.50.1.30155710 |journal=Physiological Zoology |volume=50 |issue=1 |pages=1–10 |doi=10.1086/physzool.50.1.30155710 |s2cid=82166135 |issn=0031-935X}}</ref>
====Reproductive method====
Over 97% of all known fishes are ],<ref name="Scott">Peter Scott: ''Livebearing Fishes'', p. 13. Tetra Press 1997. ISBN 1-5646-5193-2</ref> that is, the eggs develop outside the mother's body. Examples of oviparous fishes include ], ], ]s, ], and ]s. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their ]s into the surrounding water. However, a few oviparous fishes practise internal fertilisation, with the male using some sort of ] to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the ], and oviparous rays, such as ]s. In these cases, the male is equipped with a pair of modified ] fins known as ].


A few fish live mostly on land or lay their eggs on land near water.<ref>{{cite book |last=Martin |first=K.L.M. |title=Beach-Spawning Fishes: Reproduction in an Endangered Ecosystem |year=2014 |publisher=] |isbn=978-1-4822-0797-2}}</ref> ]s feed and interact with one another on mudflats and go underwater to hide in their burrows.<ref>{{FishBase |genus=Periophthalmus |species=barbarus |year=2006 |month=November |id=12803}}</ref> A single ] of '']'' has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged ].<ref>{{cite web |url=http://www.planetcatfish.com/catelog/species.php?species_id=646 |title=Cat-eLog: Heptapteridae: ''Phreatobius'': ''Phreatobius'' sp. (1) |website=Planet Catfish |access-date=26 November 2006 |url-status=live |archive-url=https://web.archive.org/web/20061023092123/http://www.planetcatfish.com/catelog/species.php?species_id=646 |archive-date=23 October 2006}}</ref><ref>{{cite journal |last1=Henderson |first1=P.A. |last2=Walker |first2=I. |year=1990 |title=Spatial organization and population density of the fish community of the litter banks within a central Amazonian blackwater stream |journal=Journal of Fish Biology |volume=37 |issue=3 |pages=401–411 |doi=10.1111/j.1095-8649.1990.tb05871.x |bibcode=1990JFBio..37..401H }}</ref> ] of multiple families live in ]s, ]s or ]s.<ref name="Helfman 2007">{{cite book |last=Helfman |first=G.S. |year=2007 |title=Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources |pages=41–42 |publisher=] |isbn=978-1-55963-595-0}}</ref>
The newly-hatched young of oviparous fish are called ]e. They are usually poorly formed, carry a large ] (from which they gain their nutrition) and are very different in appearance to juvenile and adult specimens of their species. The larval period in oviparous fish is relatively short however (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed ]) to resemble juveniles of their species. During this transition larvae use up their yolk sac and must switch from yolk sac nutrition to feeding on ] prey, a process which is dependent on zooplankton prey densities and causes many mortalities in larvae.


=== Parasites and predators ===
] fish are ones in which the eggs develop inside the mother's body after internal fertilization but receive little or no ] from the mother, depending instead on the ]. Each embryo develops in its own egg. Familiar examples of ovoviviparous fishes include ], ]s, and ]s.


{{further|Fish diseases and parasites|Predatory fish}}
Some species of fish are ]. In such species the mother retains the eggs, as in ovoviviparous fishes, but the embryos receive nutrition from the mother in a variety of different ways. Typically, viviparous fishes have a structure analogous to the ] seen in ] connecting the mother's blood supply with the that of the embryo. Examples of viviparous fishes of this type include the ], ], and ]. The embryos of some viviparous fishes exhibit a behaviour known as ] where the developing embryos eat eggs produced by the mother. This has been observed primarily among sharks, such as the ] and ], but is known for a few bony fish as well, such as the ] ''Nomorhamphus ebrardtii''.<ref name="Meisner">Meisner, A & Burns, J: Viviparity in the Halfbeak Genera ''Dermogenys'' and ''Nomorhamphus'' (Teleostei: Hemiramphidae). Journal of Morphology 234, pp 295-317, 1997</ref> ] is an even more unusual mode of vivipary, where the largest embryos in the uterus will eat their weaker and smaller siblings. This behaviour is also most commonly found among sharks, such as the ], but has also been reported for ''Nomorhamphus ebrardtii''.<ref name="Meisner">Meisner, A & Burns, J: Viviparity in the Halfbeak Genera ''Dermogenys'' and ''Nomorhamphus'' (Teleostei: Hemiramphidae). Journal of Morphology 234, pp 295-317, 1997</ref>


Like other animals, fish suffer from ]. Some species use ] to remove external parasites. The best known of these are the ]s of ]s in the ] and ] oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract the attention of the cleaners.{{sfn|Helfman|Collette|Facey|1997|p=380}} Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, '']'', the cleaner, and the much larger '']''.<ref>{{cite journal |last1=Wyman |first1=Richard L. |last2=Ward |first2=Jack A. |year=1972 |title=A Cleaning Symbiosis between the Cichlid Fishes Etroplus maculatus and Etroplus suratensis. I. Description and Possible Evolution |journal=Copeia |volume=1972 |issue=4 |pages=834–838 |doi=10.2307/1442742 |jstor=1442742 }}</ref>
] commonly refer to ovoviviparous and viviparous fishes as ].


Fish occupy many ]s in freshwater and marine ]s. Fish at the higher levels ], and a substantial part of their prey consists of other fish.<ref name="Myers Worm 2003">{{cite journal |last1=Myers |first1=Ransom A. |last2=Worm |first2=Boris |title=Rapid worldwide depletion of predatory fish communities |journal=Nature |publisher=] |volume=423 |issue=6937 |year=2003 |issn=0028-0836 |doi=10.1038/nature01610 |pages=280–283|pmid=12748640 |bibcode=2003Natur.423..280M }}</ref> In addition, mammals such as ]s and ]s feed on fish, alongside birds such as ]s and ]s.<ref>{{cite web |title=Predation |url=https://www.nwcouncil.org/fish-and-wildlife/fw-topics/predation/ |publisher=] |access-date=10 February 2024}}</ref>
===Immune system===
Types of immune organs vary between different types of fish.<ref>A.G. Zapata, A. Chiba and A. Vara. ''Cells and tissues of the immune system of fish.'' In: The Fish Immune System: Organism, Pathogen and Environment. Fish Immunology Series. (eds. G. Iwama and T.Nakanishi,), New York, Academic Press, 1996, pages 1-55.</ref>
In the ] (lampreys and hagfishes), true lymphoid organs are absent. Instead, these fish rely on regions of ] within other organs to produce their immune cells. For example, ], ] and ] are produced in the anterior kidney (or ]) and some areas of the gut (where ] mature) resemble primitive ] in hagfish.
] (sharks and rays) have a more advanced immune system than the jawless fish. They have three specialized organs that are unique to chondrichthyes; the epigonal organs (lymphoid tissue similar to bone marrow of mammals) that surround the gonads, the Leydig’s organ within the walls of their esophagus, and a spiral valve in their intestine. All these organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable ] and a well-developed ] (their most important immune organ) where various ], plasma cells and macrophages develop and are stored.
] fish (sturgeons, paddlefish and birchirs) possess a major site for the production of granulocytes within a mass that is associated with the ] (membranes surrounding the central nervous system) and their heart is frequently covered with tissue that contains lymphocytes, ] and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.
Like chondrostean fish, the major immune tissues of bony fish (or ]) include the kidney (especially the anterior kidney), where many different immune cells are housed.<ref>D.P. Anderson. ''Fish Immunology''. (S.F. Snieszko and H.R. Axelrod, eds), Hong Kong: TFH Publications, Inc. Ltd., 1977.</ref> In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.<ref>S. Chilmonczyk. ''The thymus in fish: development and possible function in the immune response''. Annual Review of Fish Diseases, Volume 2, 1992, pages 181-200.</ref><ref>J.D. Hansen and A.G. Zapata. ''Lymphocyte development in fish and amphibians''. Immunological Reviews, Volume 166, 1998, pages 199-220.</ref> Recently, a lymphatic system similar to that described in mammals was described in one species of teleost fish, the zebrafish. Although not confirmed as yet, this system presumably will be where naive (unstimulated) ] will accumulate while waiting to encounter an ].<ref>Kucher et al.,. ''Development of the zebrafish lymphatic system requires VegFc signalling''. Current Biology, Volume 16, 2006, pages 1244-1248. </ref>


<gallery mode=packed>
==Evolution==
File:Initial phase parrotfish feeding at Shaab Marsa Alam, Red Sea, Egypt -SCUBA (6336981391).jpg|A ] feeding on ] on a ]<!-- Red Sea-->
The early fossil record on fish is not very clear. It appears it was not a sufficiently successful animal early in its evolution to leave many fossils. However, this would eventually change over time as it became a dominant form of sea life and eventually branched to create land ].
File:Arothron hispidus is being cleaned by Hawaiian cleaner wrasses, Labroides phthirophagus 1.jpg|A ] removing ]s from its client, a ]
File:Barracuda with prey.jpg|A ] preying on a smaller fish
File:1031 california sealion wright odfw (35281910502).jpg|], a predatory mammal, eating a large ]
File:Cormorant with fish (cropped).jpg|] with fish prey
</gallery>


== Anatomy and physiology ==
The proliferation was apparently due to the formation of the hinged ] because jawless fish left very few descendants.<ref> EVOLUTION, ECOLOGY AND BIODIVERSITY 05-1116-3, University of Winnipeg. Retrieved ].</ref> ]s may be a rough representative of pre-jawed fish. The first jaws are found in ] fossils. It is unclear if the advantage of a hinged jaw is greater biting force, respiratory-related, or a combination.


{{main|Fish anatomy|Fish physiology}}
Some speculate that fish may have evolved from a creature similar to a coral-like ], whose larvae resemble primitive fish in some key ways. The first ancestors of fish may have ] (as some sea squirts do today), although maybe the reverse of this is case. Candidates for early fish include ] such as ], ], ], and ].


==Homeothermy== === Locomotion ===
] off ]]]
Although most fish are exclusively aquatic and ectothermic, there are exceptions to both cases. Fish from a number of different groups have evolved the capacity to live out of the water for extended periods of time. Of these ], some such as the ] can live and move about on land for up to several days.


{{Main|Fish locomotion}}
Also, certain species of fish maintain elevated body temperatures to varying degrees. Endothermic ]s (bony fishes) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (''Gasterochisma melampus''). All sharks in the family ] &ndash; shortfin mako, long fin mako, white, porbeagle, and salmon shark &ndash; are known to have the capacity for endothermy, and evidence suggests the trait exists in family ] (thresher sharks). The degree of endothermy varies from the billfish, which warm only their eyes and brain, to ] and porbeagle sharks who maintain body temperatures elevated in excess of 20 ] above ambient water temperatures. ''See also ]''. Endothermy, though metabolically costly, is thought to provide advantages such as increased contractile force of muscles, higher rates of central ] processing, and higher rates of ].


The body of a typical fish is adapted for efficient swimming by alternately contracting paired sets of ]s on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as ] like an aircraft's flaps, enabling the fish to steer in any direction.<ref name=Sfakiotakis>{{cite journal |last1=Sfakiotakis |first1=M. |last2=Lane |first2=D. M. |last3=Davies |first3=J. B. C. |date=1999 |url=http://www.mor-fin.com/Science-related-links_files/http___www.ece.eps.hw.ac.uk_Research_oceans_people_Michael_Sfakiotakis_IEEEJOE_99.pdf |title=Review of Fish Swimming Modes for Aquatic Locomotion |journal=IEEE Journal of Oceanic Engineering |volume=24 |issue=2 |pages=237–252 |doi=10.1109/48.757275 |bibcode=1999IJOE...24..237S |s2cid=17226211 |url-status=dead |archive-url=https://web.archive.org/web/20131224091124/http://www.mor-fin.com/Science-related-links_files/http___www.ece.eps.hw.ac.uk_Research_oceans_people_Michael_Sfakiotakis_IEEEJOE_99.pdf |archive-date=2013-12-24 }}</ref>
==Diseases==
Like other animals, fish can suffer from a wide variety of diseases and parasites. To prevent disease they have a variety of '''non-specific defences''' and '''specific defences'''. Non-specific defences include the skin and scales, as well as the mucus layer secreted by the ] that traps ] and inhibits their growth. Should ]s breach these defences, fish can develop an ] that increases the flow of blood to the infected region and delivers the ] that will attempt to destroy the pathogens. Specific defences are specialised responses to particular pathogens recognised by the fish's body, in other words, an ].<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 95-96, 1997, ISBN 0-86542-256-7</ref> In recent years, ] have become widely used in aquaculture and also with ornamental fish, for example the vaccines for ] in farmed ] and ] in ].<ref>R. C. Cipriano (2001), Furunculosis And Other Diseases Caused By ''Aeromonas salmonicida''. Fish Disease Leaflet 66. U.S. Department of the Interior.</ref><ref>K H Hartman et al (2004), Koi Herpes Virus (KHV) Disease. Fact Sheet VM-149. University of Florida Institute of Food and Agricultural Sciences.</ref>


<gallery mode=packed>
Some fish will also take advantage of ] for removal of external parasites. The best known of these are the ]s of the genus ''Labroides'' found on coral reefs in the ] and ]. These small fish maintain so-called "cleaning stations" where other fish, known as hosts, will congregate and perform specific movements to attract the attention of the cleaner fish.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 380, 1997, ISBN 0-86542-256-7</ref> Cleaning behaviours have been observed in a number of other fish groups, including an interesting case between two cichlids of the same genus, ''Etroplus maculatus'', the cleaner fish, and the much larger ''Etroplus suratensis'', the host.<ref>Richard L. Wyman and Jack A. Ward (1972). A Cleaning Symbiosis between the Cichlid Fishes Etroplus maculatus and Etroplus suratensis. I. Description and Possible Evolution. Copeia, Vol. 1972, No. 4, pp. 834-838.</ref>
File:Lampanyctodes hectoris (Hector's lanternfish).svg|Anatomy of a typical fish (] shown):<br />1) ] 2) ] 3) dorsal fin 4) fat fin<br />5) caudal peduncle 6) caudal fin 7) anal fin 8)&nbsp;]s 9)&nbsp;pelvic fins 10) pectoral fins
</gallery>


Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a ] that allows them to adjust their ] by increasing or decreasing the amount of gas it contains.<ref>{{cite web |title=Actinopterygii: More on Morphology |url=https://ucmp.berkeley.edu/vertebrates/actinopterygii/actinomm.html |publisher=] |access-date=10 February 2024}}</ref>
==Importance to humans==
===Economic importance===
{{main|Aquaculture}}
{{main|Fish (food)}}
{{main|Fishing}}
{{main|Fish farming}}


The ] provide protection from ]s at the cost of adding stiffness and weight.<ref name="Quan Yang Lapeyriere Schaible 2020">{{cite journal |last1=Quan |first1=Haocheng |last2=Yang |first2=Wen |last3=Lapeyriere |first3=Marine |last4=Schaible |first4=Eric |last5=Ritchie |first5=Robert O. |last6=Meyers |first6=Marc A. |title=Structure and Mechanical Adaptability of a Modern Elasmoid Fish Scale from the Common Carp |journal=Matter |volume=3 |issue=3 |date=2020 |doi=10.1016/j.matt.2020.05.011 |pages=842–863}}</ref> Fish scales are often highly reflective; this ] in the open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility.<ref name="Herring 2002">{{cite book |last=Herring |first=Peter |author-link=Peter Herring |year=2002 |title=The Biology of the Deep Ocean |publisher=] |isbn=978-0-19-854956-7 |pages=192–195}}</ref>
===Recreation===
{{main|Angling}}
{{main|Fishkeeping}}
{{main|Sport fishing}}


<gallery class=center mode=nolines widths=200px>
==Conservation==
File:Swim bladder.jpg|Gas-filled ] of a ] helps maintain neutral ].
], the world's largest fish, is classified as ].]]
File:Fish scales.jpg|Silvered ] of a ] provide protection and camouflage.
As of 2006, the ] ] describes 1,173 species of fish as being threatened with extinction.<ref>{{cite web|url=http://www.iucnredlist.org/info/tables/table1|title=Table 1: Numbers of threatened species by major groups of organisms (1996–2004)|accessdate=2006-01-18}}</ref> Included on this list are species such as ],<ref>{{cite web|url=http://www.iucnredlist.org/search/details.php/8784/summ|title=Gadus morhua|accessdate=2006-01-18}}</ref> ],<ref>{{cite web|url=http://www.iucnredlist.org/search/details.php/6149/summ|title=Cyprinodon diabolis|accessdate=2006-01-18}}</ref> ]s,<ref>{{cite web|url=http://www.iucnredlist.org/search/details.php/11375/summ|title=Latimeria chalumnae|accessdate=2006-01-18}}</ref> and ]s.<ref>{{cite web|url=http://www.iucnredlist.org/search/details.php/3855/summ|title=Carcharodon carcharias|accessdate=2006-01-18}}</ref> Because fish live underwater they are much more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small areas. For example, the ] occupies only a single 3&nbsp;m by 6&nbsp;m pool.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp. 449-450, 1997, ISBN 0-86542-256-7</ref>
</gallery>


=== Circulation ===<!--first, because Respiration section demands knowledge of the heart and circulation-->
===Overfishing===
]
In the case of edible fishes such as cod and ] a major threat is overfishing.<ref>{{cite web|url=http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/6234881.stm|title=Call to halt cod 'over-fishing'|accessdate=2006-01-18}}</ref><ref>{{cite web|url=http://news.bbc.co.uk/1/hi/world/asia-pacific/6301187.stm|title=Tuna groups tackle overfishing|accessdate=2006-01-18}}</ref> Where overfishing persists, it eventually causes the collapse of the fish population (known as a "stock") because the population cannot breed fast enough to replace the individuals removed by fishing. One well-studied example of the collapse of a fishery is the ] ''Sadinops sagax caerulues'' fishery off the coast of California. From a peak in 1937 of 790,000 ] the amount of fish landed steadily declined to a mere 24,000 tonnes in 1968, at which point the fishery stopped as no longer economically viable. Such '''commercial extinction''' does not mean that the fish itself goes extinct, merely that it can no longer sustain a profitable fishery.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 462, 1997, ISBN 0-86542-256-7</ref> The main tension between fisheries science and the fishing industry is the need to balance conservation with preserving the livelihoods of fishermen. In places such as ], ], and ] the fishing industry is a major employer, so governments have a vested interest in finding a balance between conserving fish stocks while maintaining an economic level of commercial fishing.<ref>{{cite web|url=http://news.bbc.co.uk/1/hi/uk/6112352.stm|title=UK 'must shield fishing industry'|accessdate=2006-01-18}}</ref><ref>{{cite web|url=http://news.bbc.co.uk/1/hi/world/europe/6197433.stm|title=EU fish quota deal hammered out|accessdate=2006-01-18}}</ref> On the other hand, scientists and conservations push for increasingly stringent protection for fish stocks, warning that many stocks could be wiped out within fifty years.<ref>{{cite web|url=http://www.physorg.com/news81778444.html|title=Ocean study predicts the collapse of all seafood fisheries by 2050|accessdate=2006-01-13}}</ref><ref>{{cite web|url=http://www.panda.org/about_wwf/where_we_work/europe/what_we_do/mediterranean/about/marine/bluefin_tuna/tuna_at_risk/index.cfm|title=Atlantic bluefin tuna could soon be commercially extinct|accessdate=2006-01-18}}</ref>
Fish have a ]. The ] pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through the gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart.<ref>{{cite web |title=Animal Circulatory Systems |url=https://organismalbio.biosci.gatech.edu/nutrition-transport-and-homeostasis/animal-circulatory-systems/ |publisher=] |access-date=10 February 2024}}</ref>


===Habitat destruction=== === Respiration ===
A key stress on both freshwater and marine ecosystems is habitat degradation including water ], the building of ], removal of water for use by humans, and the introduction of ] species.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 463, 1997, ISBN 0-86542-256-7</ref> An example of a fish that has become endangered because of habitat change is the ], a North American freshwater fish that living in rivers that have all been changed by human activity in a variety of different ways.<ref>{{cite web|url=http://www.mt.nrcs.usda.gov/news/factsheets/pallidsturgeon.html|title=Threatened and Endangered Species: Pallid Sturgeon ''Scaphirhynchus'' Fact Sheet|accessdate=2006-01-18}}</ref>


===Exotic species=== ==== Gills ====
Introduction of ] species has occurred in a variety of places and for many different reasons. One of the best studied (and most severe) examples was the introduction of ] into ]. Since the 1960s the Nile perch gradually exterminated the 500 species of ] fishes found only in this lake and nowhere else. Some species survive now only in captive breeding programmes, but others are probably extinct.<ref>{{cite web|url=http://www.guardian.co.uk/life/feature/story/0,,1541613,00.html|title=The little fish fight back|accessdate=2006-01-18}}</ref> ], ],<ref>{{cite web|url=http://www.washingtonpost.com/wp-dyn/articles/A16439-2002Jul2.html|title=Stop That Fish!|accessdate=2007-08-26}}</ref> ], ], ], ], and ]s are other examples of fish that have caused problems by being introduced into alien environments.


{{main|Fish gill}}
===Aquarium collecting===
{{main|Fishkeeping#Conservation_and_Science}}


Fish exchange gases using ]s on either side of the ]. Gills consist of comblike structures called filaments. Each filament contains a ] network that provides a large ] for exchanging ] and ]. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in the gills flows in the opposite direction to the water, resulting in efficient ]. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath a protective bony cover or ]. They are able to oxygenate their gills using muscles in the head.<ref name="Romer 1977">{{cite book |last1=Romer |first1=Alfred Sherwood |author1-link=Alfred Romer |last2=Parsons |first2=Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location=Philadelphia |pages=316–327 |isbn=0-03-910284-X}}</ref>
==Culture==
] of ] as a Fish]]
In the ] a "great fish" swallowed ] the ]. Legends of half-human, half-fish ]s have featured in stories like those of ] and movies like '']''.


==== Air breathing ====
Among the deities said to take the form of a fish are ] of the ], ] of various ancient ] peoples, and ] of the ]s of India. The ] symbol ] is based on a constellation of the ], but there is also a second fish constellation in the night sky, ].


{{further|amphibious fish}}
Fish have been used figuratively in many different ways, for example the ] used by early ]s to identify themselves, through to the fish as a symbol of fertility among Bengalis.<ref>Jaffrey, M.: A Taste of India, Atheneum, p 148, 1988, ISBN 0-689-70726-6</ref> Fish have also featured prominently in art and literature, as in movies such as '']'' and books such as '']''. Large fish, particularly sharks, have frequently been the subject of ] and ], most notably the novel '']'', which spawned a series of films of the ] that in turn inspired similar films or parodies such as '']'', '']'', and '']''.


Some 400 species of fish in 50 families<!--and 17 orders--> can breathe air, enabling them to live in oxygen-poor water or to emerge on to land.<ref name="Graham Wegner 2010">{{cite book |last1=Graham |first1=Jeffrey B. |last2=Wegner |first2=N.C. |chapter=6. Breathing air in water and in air: the air-breathing fishes |editor-last=Nilsson |editor-first=Göran E. |title=Respiratory Physiology of Vertebrates |publisher=] |publication-place=New York |year=2010 |isbn=978-0-521-87854-8 |pages=174–221 |doi=10.1017/CBO9780511845178.007 |url=https://www.researchgate.net/publication/263929585}}</ref> The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to the heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of ]s and the ], are facultative air breathers, able to breathe air when out of water, as may occur daily at ], and to use their gills when in water. Some coastal fish like ] and ]s choose to leave the water to feed in habitats temporarily exposed to the air.<ref name="Graham Wegner 2010"/> Some catfish <!--], ], and ]--> absorb air through their digestive tracts.<ref name="Armbruster1998">{{cite journal |last=Armbruster |first=Jonathan W. |year=1998 |title=Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes |url=http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf |url-status=dead |journal=] |volume=1998 |issue=3 |pages=663–675 |doi=10.2307/1447796 |jstor=1447796 |archive-url=https://web.archive.org/web/20090326130527/http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf |archive-date=26 March 2009 |access-date=25 June 2009}}</ref>
The golden ] (Sanskrit: Matsya), represents in the ] of ],(] symbolism) the state of fearless suspension in ], thus perceived as the harmless ], referred to as 'buddha-eyes' or ' rigpa-sight] '. The fishes symbolises the auspiciousness of all living beings in a state of fearlessness without danger of drowning in the Samsaric Ocean of Suffering, and migrating from teaching to teaching freely and spontaneously just as fish swim.


=== Digestion ===
In the following quotation, the two golden fishes are linked with the ] and ], and ], ] and carp:


The digestive system consists of a tube, the gut, leading from the mouth to the anus. The mouth of most fishes contains teeth to grip prey, bite off or scrape plant material, or crush the food. An ] carries food to the stomach where it may be stored and partially digested. A sphincter, the pylorus, releases food to the intestine at intervals. Many fish have finger-shaped pouches, ], around the pylorus, of doubtful function. The ] secretes enzymes into the intestine to digest the food; other enzymes are secreted directly by the intestine itself. The ] produces ] which helps to break up fat into an emulsion which can be absorbed in the intestine.<ref>{{cite web |title=Digestive System |url=http://web.utk.edu/~rstrange/wfs550/html-con-pages/v-digest-sys.html |publisher=] |access-date=10 February 2024}}</ref>
The two fishes originally represented the two main sacred rivers of India - the Ganges and Yamuna. These rivers are associated with the lunar and solar channels which originate in the nostrils and carry the alternating rhythms of breath & prana. They have religious significance in Hindu, Jain and Buddhist traditions but also in ] who is first signified by the ], and especially referring to ] in the desert. In the ] of Buddha the fish symbolize happiness as they have complete freedom of movement in the water. They represent fertility and abundance. Often drawn in the form of carp which are regarded in the Orient as sacred on account of their elegant beauty, size and life-span.


== Terminology == === Excretion ===
=== Fish or fishes ===
Though often used interchangeably, these words actually mean different things. '''''Fish''''' is used either as singular noun or to describe a group of specimens from a single species. '''''Fishes''''' describes a group containing more than one species.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0-86542-256-7</ref> Hence, as plurals, these words could be used thus:
* My aquarium contains three different fishes: guppies, platies, and swordtails.
* The North Atlantic stock of ''Gadus morhua'' is estimated to contain several million fish.


Most fish release their nitrogenous wastes as ]. This may be excreted through the gills or ] by the ]s. Salt is excreted by the rectal gland.<ref name="Burton Burton 2017">{{cite book |last1=Burton |first1=Derek |last2=Burton |first2=Margaret |title=Oxford Scholarship Online |chapter=Excretion |publisher=] |date=2017-12-21 |volume=1 |doi=10.1093/oso/9780198785552.003.0008}}</ref> Saltwater fish tend to lose water by ]; their kidneys return water to the body, and produce a concentrated urine. The reverse happens in ]: they tend to gain water osmotically, and produce a dilute urine. Some fish have kidneys able to operate in both freshwater and saltwater.<ref name="The Royal Society 1971">{{cite journal | title=Fish gills: mechanisms of salt transfer in fresh water and sea water | journal=Philosophical Transactions of the Royal Society of London B | volume=262 | issue=842 | date=1971-08-20 | issn=0080-4622 | doi=10.1098/rstb.1971.0090 | pages=209–249| bibcode=1971RSPTB.262..209M | last1=Maetz | first1=J. }}</ref>
=== Shoal or school ===
] are shoaling, not schooling: though swimming as a group, their speed and direction is not synchronised.]]


=== Brain ===
] are schooling because their swimming is synchronised.]]
A random assemblage of fishes merely using some localised resource such as food or nesting sites is known simply as an '''''aggregation'''''. When fish come together in an interactive, social grouping, then they may be forming either a ''shoal'' or a ''school'' depending on the degree of organisation. A '''''shoal''''' is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. '''''Schools''''' of fish are much more tightly organised, synchronising their swimming so that all fish move at the same speed and in the same direction. Shoaling and schooling behaviour is believed to provide a variety of advantages.<ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 375, 1997, ISBN 0-86542-256-7</ref>


] brain, from above|alt=Diagram showing the pairs of olfactory, telencephalon, and optic lobes, followed by the cerebellum and the mylencephalon]]
Examples:
*Cichlids congregating at ] sites form an ''aggregation''.
*Many minnows and characins form ''shoals''.
*Anchovies, herrings, and silversides are classic examples of ''schooling'' fishes.


Fish have small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal.{{sfn|Helfman|Collette|Facey|1997|pp=48–49}} However, some fish have relatively large brains, notably ] and ]s, which have brains about as large for their body weight as birds and ]s.{{sfn|Helfman|Collette|Facey|1997|p=191}} At the front of the brain are the ], a pair of structures that receive and process signals from the ]s via the two ]s. Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the ], which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to the midbrain is the ]; it works with ]s and ]. The ] is just above the diencephalon; it detects light, maintains ] rhythms, and controls color changes. The ] contains the two ]. These are very large in species that hunt by sight, such as ] and ]s. The ] controls swimming and balance.The single-lobed cerebellum is the biggest part of the brain; it is small in hagfish and ]s, but very large in ]s, processing their ]. The brain stem or ] controls some muscles and body organs, and governs respiration and ].{{sfn|Helfman|Collette|Facey|1997|pp=48–49}}
While school and shoal have different meanings within biology, they are often treated as ]s by non-specialists, with speakers of ] using "shoal" to describe any grouping of fish, while speakers of ] often using "school" just as loosely.


=== Sensory systems ===
{{further|]}}

{{main|Sensory systems in fish}}

The ] system is a network of sensors in the skin which detects gentle currents and vibrations, and senses the motion of nearby fish, whether predators or prey.<ref name="Bleckmann Zelick 2009">{{Cite journal |last1=Bleckmann |first1=Horst |last2=Zelick |first2=Randy |date=2009-03-01 |title=Lateral line system of fish |journal=] |volume=4 |issue=1 |pages=13–25 |doi=10.1111/j.1749-4877.2008.00131.x |pmid=21392273 |doi-access=free}}</ref> This can be considered both a sense of ] and of ]. ] navigate almost entirely through the sensations from their lateral line system.<ref>{{cite book |last=Godfrey-Smith |first=Peter |author-link=Peter Godfrey-Smith |title=Metazoa |publisher=] |location=New York |isbn=9780374207946 |chapter=Kingfish |date=2020 }}</ref> Some fish, such as catfish and sharks, have the ], ]s that detect weak electric currents on the order of millivolt.<ref name="Albert Crampton 2006">{{cite book |last1=Albert |first1=J. S. |last2=Crampton |first2=W. G. |year=2006 |chapter=Electroreception and Electrogenesis |editor=Lutz, P. L. |title=The Physiology of Fishes |publisher=] |location=Boca Raton, Florida |isbn=978-0-8493-2022-4 |pages=429–470}}</ref>

] is an important ] in fish.<ref name="Guthrie 1986">{{cite book |last=Guthrie |first=D. M. |title=The Behaviour of Teleost Fishes |chapter=Role of Vision in Fish Behaviour |publisher=] |publication-place=Boston, Massachusetts |date=1986 |isbn=978-1-4684-8263-8 |doi=10.1007/978-1-4684-8261-4_4 |pages=75–113}}</ref> Fish eyes are similar to those of ] ]s like ] and mammals, but have a more ] ].<ref name="Guthrie 1986"/> Their ]s generally have both ] and ] (for ] and ]); many species have ], often with three types of cone.<ref name="Guthrie 1986"/> Teleosts can see ];<ref name="Hawryshyn 2010">{{cite journal |last=Hawryshyn |first=Craig W. |title=Ultraviolet Polarization Vision and Visually Guided Behavior in Fishes |journal=Brain, Behavior and Evolution |volume=75 |issue=3 |date=2010 |issn=0006-8977 |doi=10.1159/000314275 |pages=186–194 |pmid=20733294 }}</ref> some such as cyprinids have a fourth type of cone that detects ].<ref name="Guthrie 1986"/> Amongst ], the ] has well-developed eyes,<ref name="Meyer-Rochow 1996">{{cite journal |last1=Meyer-Rochow |first1=V. Benno |last2=Stewart |first2=Duncan |title=Review of larval and postlarval eye ultrastructure in the lamprey (cyclostomata) with special emphasis on Geotria australis (gray) |journal=Microscopy Research and Technique |date=1996 |volume=35 |issue=6 |pages=431–444 |doi=10.1002/(SICI)1097-0029(19961215)35:6<431::AID-JEMT3>3.0.CO;2-L |pmid=9016447 |s2cid=22940203 }}</ref> while the ] has only primitive eye<!--don't wikilink, wrong subject-->spots.<ref name="Lamb Collin Pugh 2007">{{cite journal |last1=Lamb |first1=Trevor D. |last2=Collin |first2=Shaun P. |last3=Pugh |first3=Edward N. |title=Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup |journal=Nature Reviews Neuroscience |volume=8 |issue=12 |date=2007 |issn=1471-003X |pmid=18026166 |pmc=3143066 |doi=10.1038/nrn2283 |pages=960–976}} See also </ref>

] too is an important sensory system in fish. Fish sense sound using their lateral lines and ]s in their ears, inside their heads. Some can detect sound through the swim bladder.<ref name="Hawkins 1981">{{cite book |last=Hawkins |first=A. D. |editor1-last=Tavolga |editor1-first=William N. |editor2-last=Popper |editor2-first=Arthur N. |editor3-last=Fay |editor3-first=Richard R. |title=Hearing and Sound Communication in Fishes |date=1981 |publisher=] |isbn=978-1-4615-7188-9 |pages=109–138 |chapter=6. The Hearing Abilities of Fish |url=https://books.google.com/books?id=egP3BwAAQBAJ}}</ref>

Some fish, including salmon, are capable of ]; when the axis of a magnetic field is changed around a circular tank of young fish, they reorient themselves in line with the field.<ref name="Quinn 1980">{{cite journal |last=Quinn |first=Thomas P. |date=1980 |title=Evidence for celestial and magnetic compass orientation in lake migrating sockeye salmon fry |journal=] |volume=137 |issue=3 |pages=243–248 |doi=10.1007/bf00657119 |s2cid=44036559 }}</ref><ref name="Taylor 1986">{{Cite journal |last=Taylor |first=P. B. |date=May 1986 |title=Experimental evidence for geomagnetic orientation in juvenile salmon, Oncorhynchus tschawytscha Walbaum |journal=] |volume=28 |issue=5 |pages=607–623 |doi=10.1111/j.1095-8649.1986.tb05196.x |bibcode=1986JFBio..28..607T }}</ref> The mechanism of fish magnetoreception remains unknown;<ref name="Formicki Korzelecka‐Orkisz Tański 2019">{{cite journal |last1=Formicki |first1=Krzysztof |last2=Korzelecka-Orkisz |first2=Agata |last3=Tański |first3=Adam |title=Magnetoreception in fish |journal=Journal of Fish Biology |volume=95 |issue=1 |date=2019 |issn=0022-1112 |doi=10.1111/jfb.13998 |pages=73–91|pmid=31054161 |bibcode=2019JFBio..95...73F }}</ref> experiments in birds imply a quantum ].<ref name="Hore Mouritsen 2022">{{cite journal |last1=Hore |first1=Peter J. |author1-link=Peter Hore (chemist) |last2=Mouritsen |first2=Henrik |title=The Quantum Nature of Bird Migration |journal=] |url=https://www.scientificamerican.com/article/how-migrating-birds-use-quantum-effects-to-navigate/ |date=April 2022 |pages=24–29}}</ref>

=== Cognition ===

{{further|Fish intelligence}}

The cognitive capacities of fish include ], as seen in ]s. ] and ]s placed in front of a mirror repeatedly check whether their reflection's behavior mimics their body movement.<ref>{{Cite journal |last1=Ari |first1=Csilla |last2=D'Agostino |first2=Dominic P. |date=2016-05-01 |title=Contingency checking and self-directed behaviors in giant manta rays: Do elasmobranchs have self-awareness? |url=https://doi.org/10.1007/s10164-016-0462-z |journal=Journal of Ethology |volume=34 |issue=2 |pages=167–174 |doi=10.1007/s10164-016-0462-z |s2cid=254134775 |access-date=21 January 2023 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010603/https://link.springer.com/article/10.1007/s10164-016-0462-z |url-status=live}}</ref><ref>{{Cite journal |last1=Kohda |first1=Masanori |last2=Hotta |first2=Takashi |last3=Takeyama |first3=Tomohiro |last4=Awata |first4=Satoshi |last5=Tanaka |first5=Hirokazu |last6=Asai |first6=Jun-ya |last7=Jordan |first7=L. Alex |date=2018-08-21 |journal=] |title=Cleaner wrasse pass the mark test. What are the implications for consciousness and self-awareness testing in animals? |volume=17 |issue=2 |pages=397067 |doi=10.1371/journal.pbio.3000021|doi-access=free |pmid=30730878 |pmc=6366756 |biorxiv=10.1101/397067 |s2cid=91375693}}</ref> '']'' wrasse, ], and ] can solve problems and invent tools.<ref>{{cite magazine |last=Balcombe |first=Jonathan |title=Fishes Use Problem-Solving and Invent Tools |magazine=] |date=1 May 2017 |url=https://www.scientificamerican.com/article/fishes-use-problem-solving-and-invent-tools1/ |archive-url=https://web.archive.org/web/20230317010617/https://www.scientificamerican.com/article/fishes-use-problem-solving-and-invent-tools1/ |url-status=live |archive-date=17 March 2023}}</ref> The ] cichlid '']'' exhibits pessimistic behavior when prevented from being with its partner.<ref>{{cite journal |first1=Chloé |last1=Laubu |first2=Philippe |last2=Louâpre |first3=François-Xavier |last3=Dechaume-Moncharmont |date=2019 |title=Pair-bonding influences affective state in a monogamous fish species |journal=Proceedings of the Royal Society B |volume=286 |issue=1904 |at=20190760 |doi=10.1098/rspb.2019.0760 |doi-access=free |pmid=31185864 |pmc=6571461}}</ref> Fish orient themselves using landmarks; they may use mental maps based on multiple landmarks. Fish are able to learn to traverse mazes, showing that they possess spatial memory and visual discrimination.<ref>{{cite web |last=Sciences |first=Journal of Undergraduate Life |url=http://juls.sa.utoronto.ca/Issues/JULS-Vol2Iss1/JULS-Vol2Iss1-Review3.pdf |title=Appropriate maze methodology to study learning in fish |access-date=28 May 2009 |url-status=dead |archive-url=https://web.archive.org/web/20110706211428/http://juls.sa.utoronto.ca/Issues/JULS-Vol2Iss1/JULS-Vol2Iss1-Review3.pdf |archive-date=6 July 2011}}</ref> Behavioral research suggests that fish are ], capable of experiencing ].<ref>{{Cite web |last=Woodruff |first=Michael |date=3 July 2020 |title=The face of the fish |url=https://aeon.co/essays/fish-are-nothing-like-us-except-that-they-are-sentient-beings |access-date=28 July 2024 |website=]}}</ref>

=== Electrogenesis ===

] is a weakly electric fish which generates an ] with its ] and then uses its ] to locate objects by the distortions they cause in its electric field.<ref name="von der Emde 1999">{{cite journal |last=von der Emde |first=G. |title=Active electrolocation of objects in weakly electric fish |journal=] |volume=202 |issue=10 |date=15 May 1999 |doi=10.1242/jeb.202.10.1205 |pages=1205–1215|pmid=10210662 }}</ref>]]

{{further|Electroreception and electrogenesis}}

] such as ], the ], and ] have some of their muscles adapted to ]. They use the field to locate and identify objects such as prey in the waters around them, which may be turbid or dark.<ref name="Albert Crampton 2006"/> Strongly electric fish like the electric eel can in addition use their ] to generate shocks powerful enough to stun their prey.<ref name="Catania 2015 high-voltage">{{cite journal |last1=Catania |first1=Kenneth C. |title=Electric eels use high-voltage to track fast-moving prey |journal=Nature Communications |date=20 October 2015 |volume=6 |page=8638 |doi=10.1038/ncomms9638 |pmid=26485580 |pmc=4667699 |bibcode=2015NatCo...6.8638C}}</ref>

=== Endothermy ===

Most fish are exclusively cold-blooded or ]. However, the ] are ] (endothermic), including the ]es and tunas.<ref name="Block1993">{{cite journal |last1=Block |first1=B.A. |last2=Finnerty |first2=JR |year=1993 |title=Endothermy in fishes: a phylogenetic analysis of constraints, predispositions, and selection pressures |url=https://www.researchgate.net/publication/226138605 |format=PDF |journal=Environmental Biology of Fishes |volume=40 |issue=3 |pages=283–302 |doi=10.1007/BF00002518 |s2cid=28644501 |access-date=1 October 2018 |archive-date=6 November 2020 |archive-url=https://web.archive.org/web/20201106144640/https://www.researchgate.net/publication/226138605 |url-status=live }}</ref> The ], a ], uses whole-body endothermy, generating heat with its swimming muscles to warm its body while countercurrent exchange minimizes heat loss.<ref>{{Cite journal |last1=Wegner |first1=Nicholas C. |last2=Snodgrass |first2=Owyn E. |last3=Dewar |first3=Heidi |last4=Hyde |first4=John R. |date=2015-05-15 |title=Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus |journal=Science |volume=348 |issue=6236 |pages=786–789 |doi=10.1126/science.aaa8902 |issn=0036-8075 |pmid=25977549 |bibcode=2015Sci...348..786W |s2cid=17412022 }}</ref> Among the cartilaginous fishes, sharks of the families ] (such as the great white shark) and ] (thresher sharks) are endothermic. The degree of endothermy varies from the billfishes, which warm only their eyes and brain, to the ] and the ], which maintain body temperatures more than {{convert|20|C}} above the ambient water.<ref name="Block1993"/><ref>{{cite journal |last=Goldman |first=K.J. |title=Regulation of body temperature in the white shark, ''Carcharodon carcharias'' |journal=Journal of Comparative Physiology |year=1997 |volume=167 |series=B Biochemical Systemic and Environmental Physiology |issue=6 |pages=423–429 |doi=10.1007/s003600050092 |s2cid=28082417 |url=http://www.mendeley.com/research/temperature-and-activities-of-a-white-shark-carcharodon-carcharias/ |access-date=12 October 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120406114844/http://www.mendeley.com/research/temperature-and-activities-of-a-white-shark-carcharodon-carcharias/ |archive-date=6 April 2012}}</ref><ref>{{cite journal |last1=Carey |first1=F.G. |last2=Lawson |first2=K.D. |title=Temperature regulation in free-swimming bluefin tuna |journal=Comparative Biochemistry and Physiology A |date=February 1973 |volume=44 |issue=2 |pages=375–392 |doi=10.1016/0300-9629(73)90490-8 |pmid=4145757 }}</ref>

{{Anchor|Reproductive method}}

=== Reproduction and life-cycle ===

{{main|Fish reproduction}}

] fry hatching from the egg, keeping its ] ]]

The primary reproductive organs are paired ]s and ].<ref name="Guimaraes">{{cite journal |last1=Guimaraes-Cruz |first1=Rodrigo J. |author2=dos Santos, José E. |author3=Santos, Gilmar B. |title=Gonadal structure and gametogenesis of ''Loricaria lentiginosa'' Isbrücker (Pisces, Teleostei, Siluriformes) |journal=Rev. Bras. Zool. |date=July–September 2005 |volume=22 |issue=3 |pages=556–564 |issn=0101-8175 |doi=10.1590/S0101-81752005000300005 |doi-access=free }}</ref> Eggs are released from the ovary<!--directly or indirectly--> to the ]s.<ref name="Brito">{{cite journal |title=Reproduction of the surubim catfish (Pisces, Pimelodidae) in the São Francisco River, Pirapora Region, Minas Gerais, Brazil |last=Brito |first=M.F.G. |author2=Bazzoli, N. |journal=Arquivo Brasileiro de Medicina Veterinária e Zootecnia |volume=55 |issue=5 |year=2003 |doi=10.1590/S0102-09352003000500018 |issn=0102-0935 |pages=624–633 |doi-access=free }}</ref> Over 97% of fish, including salmon and goldfish, are ], meaning that the eggs are shed into the water and develop outside the mother's body.<ref name="Scott">{{cite book |last=Scott |first=Peter |title=Livebearing Fishes |page=13 |publisher=Tetra Press |year=1997 |isbn=1-56465-193-2}}</ref> The eggs are usually fertilized outside the mother's body, with the male and female fish shedding their ]s into the surrounding water. In a few oviparous fish, such as the ], fertilization is internal: the male uses an ] to deliver sperm into the female's genital opening of the female.<ref name="Miller Kendall 2009"/> Marine fish release large numbers of small eggs into the open water column. Newly hatched young of oviparous fish are ]. They have a large ] and do not resemble juvenile or adult fish. The larval period in oviparous fish is usually only some weeks, and larvae rapidly grow and ] to become juveniles. During this transition, larvae must switch from their yolk sac to feeding on ] prey.<ref name="Miller Kendall 2009"/> Some fish such as ], ], and ]s are ] or live-bearing, meaning that the mother retains the eggs and nourishes the embryos via a structure analogous to the ] to connect the mother's blood supply with the embryo's.<ref name="Miller Kendall 2009">{{cite book |last1=Miller |first1=Bruce |last2=Kendall |first2=Arthur W. |title=Early Life History of Marine Fishes |date=2009 |publisher=] |isbn=9780520249721 |pages=11–37 |chapter-url=https://content.ucpress.edu/chapters/9317.ch01.pdf |url-status=live |archive-url=https://web.archive.org/web/20160307042241/https://content.ucpress.edu/chapters/9317.ch01.pdf |archive-date=7 March 2016 |chapter=1. Fish Reproduction}}</ref>

=== DNA repair ===

Embryos of externally fertilized fish species are directly exposed during their development to environmental conditions that may ], such as pollutants, ] and ].<ref name="Dey2023">Dey A, Flajšhans M, Pšenička M, Gazo I. DNA repair genes play a variety of roles in the development of fish embryos. Front Cell Dev Biol. 2023 Mar 1;11:1119229. doi: 10.3389/fcell.2023.1119229. PMID 36936683; PMCID: PMC10014602</ref> To deal with such DNA damages, a variety of different ] pathways are employed by fish embryos during their development.<ref name = Dey2023/> In recent years ] have become a useful model for assessing environmental pollutants that might be genotoxic, i.e. cause DNA damage.<ref>Canedo A, Rocha TL. Zebrafish (Danio rerio) using as model for genotoxicity and DNA repair assessments: Historical review, current status and trends. Sci Total Environ. 2021 Mar 25;762:144084. doi: 10.1016/j.scitotenv.2020.144084. Epub 2020 Dec 14. PMID 33383303</ref>

=== Defenses against disease ===

{{further|Immune system}}

Fish have both non-specific and immune defenses against disease. Non-specific defenses include the skin and scales, as well as the mucus layer secreted by the ] that traps and inhibits the growth of ]s. If ]s breach these defenses, the ] can mount an ] that increases blood flow to the infected region and delivers ] that attempt to destroy pathogens, non-specifically. Specific defenses respond to particular antigens, such as ]s on the surfaces of ], recognised by the ].{{sfn |Helfman |Collette |Facey |1997 |pp=95–96}} Immune systems evolved in ]s as shown in the cladogram.<ref name="Flajnik Kasahara 2010"/>

{{clade|style=font-size:95%;line-height:110%;
|label1=]s
|sublabel1= &nbsp; ''']''' &nbsp;
|1={{clade
|1= &nbsp; ]s, ]s, ]s, ]s
|label2= &nbsp; ]
|2={{clade
|sublabel1= &nbsp; ''']''' &nbsp;
|1= &nbsp; ]
|sublabel2= &nbsp;''']''' &nbsp;
|2= &nbsp; ]
}}
}}
}}

Immune organs vary by type of fish. The jawless fish have ] within the ], and ]s in the gut. They have ]; it makes use of ]s (VLR) to generate immunity to a wide range of antigens, The result is much like that of jawed fishes and tetrapods, but it may have ].<ref name="Flajnik Kasahara 2010"/> All jawed fishes have an ] with B and T ] bearing ]s and ]s respectively. This makes use of ] (V(D)J) to create immunity to a wide range of antigens. This system evolved once and is basal to the jawed vertebrate clade.<ref name="Flajnik Kasahara 2010">{{cite journal |last1=Flajnik |first1=M. F. |last2=Kasahara |first2=M. |year=2010 |title=Origin and evolution of the adaptive immune system: genetic events and selective pressures |journal=Nature Reviews Genetics |volume=11 |issue=1 |pages=47–59 |doi=10.1038/nrg2703 |pmid=19997068 |pmc=3805090}}</ref> Cartilaginous fish have three specialized organs that contain immune system cells: the epigonal organs around the gonads, ] within the esophagus, and a ] in their intestine, while their ] and ] have similar functions to those of the same organs in the immune systems of tetrapods.<ref>{{cite book |last1=Zapata |first1=A.G. |last2=Chiba |first2=A. |last3=Vara |first3=A. |chapter=Cells and tissues of the immune system of fish |title=The Fish Immune System: Organism, Pathogen and Environment |series=Fish Immunology |editor1=Iwama, G. Iwama |editor2=Nakanishi, T. |location=New York |publisher=Academic Press |year=1996 |pages=1–55}}</ref> Teleosts have lymphocytes in the thymus, and other immune cells in the spleen and other organs.<ref>{{cite journal |last=Chilmonczyk |first=S. |title=''The thymus in fish: development and possible function in the immune response'' |journal=] |volume=2 |year=1992 |pages=181–200 |doi=10.1016/0959-8030(92)90063-4}}</ref><ref>{{cite journal |last1=Hansen |first1=J.D. |last2=Zapata |first2=A.G. |year=1998 |title=''Lymphocyte development in fish and amphibians'' |journal=Immunological Reviews |volume=166 |pages=199–220 |doi=10.1111/j.1600-065x.1998.tb01264.x |pmid=9914914 |s2cid=7965762 }}</ref>

== Behavior ==

=== Shoaling and schooling ===

{{main|Shoaling and schooling}}

]es ] for safety from predators, and to spawn.<ref name="Pitcher 1986"/>]]

A ''shoal'' is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. A ''school'' is a much more tightly organised group, synchronising its swimming so that all fish move at the same speed and in the same direction.{{sfn|Helfman|Collette|Facey|1997|p=375}} Schooling is sometimes an ], offering improved vigilance against predators. It is often more efficient to gather food by working as a group, and individual fish optimise their strategies by choosing to join or leave a shoal. When a predator has been noticed, prey fish respond defensively, resulting in collective shoal behaviours such as synchronised movements. Responses do not consist only of attempting to hide or flee; antipredator tactics include for example scattering and reassembling. Fish also aggregate in shoals to spawn.<ref name="Pitcher 1986">{{cite book |last=Pitcher |first=Tony J. |chapter=12. Functions of Shoaling Behaviour in Teleosts |title=The Behaviour of Teleost Fishes |publisher=Springer |year=1986 |pages=294–337 |doi=10.1007/978-1-4684-8261-4_12 |isbn=978-1-4684-8263-8}}</ref> The ] migrates annually in large schools between its feeding areas and its spawning grounds.<ref name="Gjøsæter 1998">{{cite journal |last=Gjøsæter |first=H. |title=The population biology and exploitation of capelin (''Mallotus villosus'') in the Barents Sea |journal=Sarsia |year=1998 |volume=83 |issue=6 |pages=453–496 |doi=10.1080/00364827.1998.10420445 |url=https://www.researchgate.net/publication/230660010}}</ref>

=== Communication ===

{{see also|Acoustic communication in aquatic animals}}

Fish communicate by transmitting acoustic signals (sounds) to each other. This is most often in the context of feeding, aggression or courtship.<ref name="Bioacoustics">{{cite journal |last1=Weinmann |first1=S.R. |last2=Black |first2=A.N. |last3=Richter |first3=M.L. |last4=Itzkowitz |first4=M. |last5=Burger |first5=R.M. |title=Territorial vocalization in sympatric damselfish: acoustic characteristics and intruder discrimination |journal=Bioacoustics |date=February 2017 |volume=27 |issue=1 |pages=87–102 |doi=10.1080/09524622.2017.1286263 |s2cid=89625932}}</ref> The sounds emitted vary with the species and stimulus involved. Fish can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.<ref name="Journal">{{cite journal |last1=Bertucci |first1=F. |last2=Ruppé |first2=L. |last3=Wassenbergh |first3=S.V. |last4=Compère |first4=P. |last5=Parmentier |first5=E. |date=29 October 2014 |title=New Insights into the Role of the Pharyngeal Jaw Apparatus in the Sound-Producing Mechanism of Haemulon Flavolineatum (Haemulidae) |journal=Journal of Experimental Biology |volume=217 |issue=21 |pages=3862–3869 |doi=10.1242/jeb.109025 |pmid=25355850 |doi-access=free |hdl=10067/1197840151162165141|hdl-access=free}}</ref>

] makes sounds by grinding its teeth. ]]

Some fish produce sounds by rubbing or grinding their bones together. These sounds are stridulatory. In '']'', the French grunt fish, as it produces a grunting noise by grinding its teeth together, especially when in distress. The grunts are at a frequency of around 700&nbsp;Hz, and last approximately 47 milliseconds.<ref name="Journal"/> The longsnout seahorse, '']'' produces two categories of sounds, 'clicks' and 'growls', by rubbing their coronet bone across the grooved section of their neurocranium.<ref>{{cite journal |last1=Colson |first1=D.J. |last2=Patek |first2=S.N. |last3=Brainerd |first3=E.L. |last4=Lewis |first4=S.M. |title=Sound production during feeding in Hippocampus seahorses (Syngnathidae) |journal=Environmental Biology of Fishes |date=February 1998 |volume=51 |issue=2 |pages=221–229 |doi=10.1023/A:1007434714122 |bibcode=1998EnvBF..51..221C |s2cid=207648816}}</ref> Clicks are produced during courtship and feeding, and the frequencies of clicks were within the range of 50&nbsp;Hz-800&nbsp;Hz. The frequencies are at the higher end of the range during spawning, when the female and male fishes were less than fifteen centimeters apart. Growls are produced when the ''H. reidi'' are stressed. The 'growl' sounds consist of a series of sound pulses and are emitted simultaneously with body vibrations.<ref name="Zoology">{{cite journal |last1=Oliveira |first1=T.P.R. |last2=Ladich |first2=F. |last3=Abed-Navandi |first3=D. |last4=Souto |first4=A.S. |last5=Rosa |first5=I.L. |title=Sounds produced by the longsnout seahorse: a study of their structure and functions|journal=Journal of Zoology|date=26 June 2014|volume=294 |issue=2 |pages=114–121 |doi=10.1111/jzo.12160}}</ref>

Some fish species create noise by engaging specialized muscles that contract and cause swimbladder vibrations. ] produce loud grunts by contracting sonic muscles along the sides of the swim bladder.<ref name="Experimental">{{cite journal|last1=Fine |first1=L.F. |last2=King |first2=C.B. |last3=Cameron |first3=T.M. |title=Acoustical properties of the swimbladder in the oyster toadfish ''Opsanus tau'' |journal=Journal of Experimental Biology|date=16 October 2009 |volume=212 |issue=21 |pages=3542–3552|doi=10.1242/jeb.033423|pmid=19837896|pmc=2762879}}</ref> Female and male toadfishes emit short-duration grunts, often as a fright response.<ref>{{cite journal |last1=Fine |first1=M.L. |last2=Waybright |first2=T.D. |title=Grunt variation in the oyster toadfish Opsanus tau:effect of size and sex |journal=PeerJ |date=15 October 2015 |volume=3 |issue=1330 |pages=e1330 |doi=10.7717/peerj.1330 |pmid=26623178 |pmc=4662586 |doi-access=free }}</ref> In addition to short-duration grunts, male toadfishes produce "boat whistle calls".<ref name="plos" /> These calls are longer in duration, lower in frequency, and are primarily used to attract mates.<ref name="plos" /> The various sounds have frequency range of 140&nbsp;Hz to 260&nbsp;Hz.<ref name="plos">{{cite journal|last1=Ricci |first1=S.W. |last2=Bohnenstiehl |first2=D. R. |last3=Eggleston |first3=D.B. |last4=Kellogg |first4=M.L. |last5=Lyon |first5=R.P. |title=Oyster toadfish (Opsanus tau) boatwhistle call detection and patterns within a large-scale oyster restoration site |journal=PLOS ONE |date=8 August 2017 |volume=12 |issue=8 |pages=e0182757 |doi=10.1371/journal.pone.0182757|pmid=28792543|pmc=5549733 |bibcode=2017PLoSO..1282757R |doi-access=free}}</ref> The frequencies of the calls depend on the rate at which the sonic muscles contract.<ref>{{cite journal |last1=Skoglund |first1=C.R. |title=Functional analysis of swimbladder muscles engaged in sound productivity of the toadfish |journal=Journal of Cell Biology |date=1 August 1961 |volume=10 |issue=4 |pages=187–200 |doi=10.1083/jcb.10.4.187 |pmid=19866593 |pmc=2225107}}</ref><ref name="Experimental"/>

The red drum, '']'', produces drumming sounds by vibrating its swimbladder. Vibrations are caused by the rapid contraction of sonic muscles that surround the dorsal aspect of the swimbladder. These vibrations result in repeated sounds with frequencies from 100 to >200&nbsp;Hz. ''S. ocellatus'' produces different calls depending on the stimuli involved, such as courtship or a predator's attack. Females do not produce sounds, and lack sound-producing (sonic) muscles.<ref name="Elsevier">{{cite journal|last1=Parmentier|first1=E. |last2=Tock|first2=J. |last3=Falguière|first3=J.C. |last4=Beauchaud|first4=M.|title=Sound production in Sciaenops ocellatus: Preliminary study for the development of acoustic cues in aquaculture |journal=Aquaculture |date=22 May 2014|volume=432|pages=204–211|doi=10.1016/j.aquaculture.2014.05.017|bibcode=2014Aquac.432..204P |url=https://archimer.ifremer.fr/doc/00190/30127/28589.pdf|access-date=21 January 2019|archive-date=3 June 2021|archive-url=https://web.archive.org/web/20210603172200/https://archimer.ifremer.fr/doc/00190/30127/28589.pdf|url-status=live}}</ref>

== Conservation ==

The 2024 ] ] names 2,168 fish species that are endangered or critically endangered.<ref>{{cite web |url=https://www.iucnredlist.org/search?query=Fishes&searchType=species |title=Search for 'Fishes' (Global, CR-Critically Endangered, En-Endangered, Species) |access-date=27 February 2024}}</ref> Included are species such as ],<ref>{{cite iucn |last=Sobel |first=J. |date=1996 |title=''Gadus morhua'' |volume=1996 |page=e.T8784A12931575 |doi=10.2305/IUCN.UK.1996.RLTS.T8784A12931575.en |access-date=11 November 2021}}</ref> ],<ref>{{cite iucn |date=2014 |title=''Cyprinodon diabolis'' |volume=2014 |page=e.T6149A15362335 |doi=10.2305/IUCN.UK.2014-3.RLTS.T6149A15362335.en |access-date=11 November 2021}}</ref> ]s,<ref>{{cite iucn |last=Musick |first=J.A. |date=2000 |title=''Latimeria chalumnae'' |volume=2000 |page=e.T11375A3274618 |doi=10.2305/IUCN.UK.2000.RLTS.T11375A3274618.en |access-date=11 November 2021}}</ref> and ]s.<ref>{{cite iucn |last1=Rigby |first1=C.L. |last2=Barreto |first2=R. |last3=Carlson |first3=J. |last4=Fernando |first4=D. |last5=Fordham |first5=S. |last6=Francis |first6=M.P. |last7=Herman |first7=K. |last8=Jabado |first8=R.W. |last9=Liu |first9=K.M. |last10=Lowe |first10=C.G. |last11=Marshall |first11=A. |last12=Pacoureau |first12=N. |last13=Romanov |first13=E. |last14=Sherley |first14=R.B. |last15=Winker |first15=H. |display-authors=6 |year=2019 |title=''Carcharodon carcharias'' |page=e.T3855A2878674 |access-date=19 December 2019}}</ref> Because fish live underwater they are more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the Devil's Hole pupfish occupies only a single {{convert|3|by|6|m|0}} pool.{{sfn|Helfman|Collette|Facey|1997|pp=449–450}}

=== Overfishing ===

{{main|Overfishing}}

]<ref name="Hamilton 2001"/> ]]

The ] reports that "in 2017, 34 percent of the fish stocks of the world's marine fisheries were classified as overfished".<ref name="FAO 2020">{{Cite book |url=http://www.fao.org/documents/card/en/c/ca9229en |title=The State of World Fisheries and Aquaculture 2020 |date=2020 |publisher=] |isbn=978-92-5-132692-3 |doi=10.4060/ca9229en |hdl=10535/3776 |s2cid=242949831 |page=54}}</ref> Overfishing is a major threat to edible fish such as cod and ].<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/6234881.stm |title=Call to halt cod 'over-fishing' |access-date=18 January 2006 |work=BBC News |date=5 January 2007 |url-status=live |archive-url=https://web.archive.org/web/20070117142906/http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/6234881.stm |archive-date=17 January 2007}}</ref><ref>{{cite news |url=http://news.bbc.co.uk/1/hi/world/asia-pacific/6301187.stm |title=Tuna groups tackle overfishing |access-date=18 January 2006 |work=BBC News |date=26 January 2007 |url-status=live |archive-url=https://web.archive.org/web/20090121111204/http://news.bbc.co.uk/1/hi/world/asia-pacific/6301187.stm |archive-date=21 January 2009}}</ref> Overfishing eventually causes ] to collapse, because the survivors cannot produce enough young to replace those removed. Such commercial extinction does not mean that the species is extinct, merely that it can no longer sustain a fishery. In the case of the ] fishery off the California coast, the catch steadily declined from a 1937 peak of 800,000 tonnes to an economically inviable 24,000 tonnes in 1968.{{sfn|Helfman|Collette|Facey|1997|p=462}} In the ], overfishing reduced the fish population to 1% of its historical level by 1992.<ref name="Hamilton 2001">{{cite journal |last1=Hamilton |first1=Lawrence C. |last2=Butler |first2=M. J. |title=Outport adaptations: Social indicators through Newfoundland's Cod crisis |journal=Human Ecology Review |date=January 2001 |volume=8 |issue=2|pages=1–11 |url=https://www.researchgate.net/publication/247089244}}</ref>
] and the ] have sharply differing views on the resiliency of fisheries to intensive fishing. In many coastal regions the fishing industry is a major employer, so governments are predisposed to support it.<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/uk/6112352.stm |title=UK 'must shield fishing industry' |access-date=18 January 2006 |work=BBC News |date=3 November 2006 |url-status=live |archive-url=https://web.archive.org/web/20061130055349/http://news.bbc.co.uk/1/hi/uk/6112352.stm |archive-date=30 November 2006}}</ref><ref>{{cite news |url=http://news.bbc.co.uk/1/hi/world/europe/6197433.stm |title=EU fish quota deal hammered out |access-date=18 January 2006 |work=BBC News |date=21 December 2006 |url-status=live |archive-url=https://web.archive.org/web/20061226234953/http://news.bbc.co.uk/1/hi/world/europe/6197433.stm |archive-date=26 December 2006}}</ref> On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be destroyed within fifty years.<ref>{{cite web|title=Ocean study predicts the collapse of all seafood fisheries by 2050|url=http://www.physorg.com/news81778444.html|url-status=live|archive-url=https://web.archive.org/web/20070315014325/http://www.physorg.com/news81778444.html|archive-date=15 March 2007|access-date=13 January 2006|website=phys.org}}</ref><ref>{{cite web |url=http://www.panda.org/about_wwf/where_we_work/europe/what_we_do/mediterranean/about/marine/bluefin_tuna/tuna_at_risk/index.cfm |title=Atlantic bluefin tuna could soon be commercially extinct |publisher=WWF |access-date=18 January 2006 |archive-url=https://web.archive.org/web/20070430205610/http://www.panda.org/about_wwf/where_we_work/europe/what_we_do/mediterranean/about/marine/bluefin_tuna/tuna_at_risk/index.cfm |archive-date=30 April 2007 |url-status=dead}}</ref>

=== Other threats ===

A key stress on both freshwater and marine ecosystems is ] including ], the building of dams, removal of water for use by humans, and the introduction of ] species including predators.{{sfn|Helfman|Collette|Facey|1997|p=463}} Freshwater fish, especially if ] to a region (occurring nowhere else), may be threatened with extinction for all these reasons, as is the case for three of Spain's ten endemic freshwater fishes.<ref name="Elvira 1995">{{cite journal |last=Elvira |first=Benigno |title=Conservation status of endemic freshwater fish in Spain |journal=Biological Conservation |publisher=Elsevier |volume=72 |issue=2 |year=1995 |issn=0006-3207 |doi=10.1016/0006-3207(94)00076-3 |pages=129–136|bibcode=1995BCons..72..129E }}</ref> River dams, especially major schemes like the ] (Zambezi river) and the ] (]) on rivers with economically important fisheries, have caused large reductions in fish catch.<ref>{{cite book |last1=Jackson |first1=Donald C. |last2=Marmulla |first2=Gerd |title=The influence of dams on river fisheries |publisher=FAO Fisheries |volume=Technical paper 419 |date=2001 |pages=1–44 |url=http://www.friendsofmerrymeetingbay.org/cybrary/pages/20010000_UN_FAO_Dams,%20fish%20and%20fisheries.pdf#page=6}}</ref> Industrial bottom trawling ], as has occurred on the ] in the North Atlantic.<ref name="Duplisea Frisk Trenkel 2016">{{cite journal |last1=Duplisea |first1=Daniel E. |last2=Frisk |first2=Michael G. |last3=Trenkel |first3=Verena M. |title=Extinction Debt and Colonizer Credit on a Habitat Perturbed Fishing Bank |journal=PLOS ONE |publisher=Public Library of Science (PLoS) |volume=11 |issue=11 |date=2016-11-28 |issn=1932-6203 |doi=10.1371/journal.pone.0166409 |doi-access=free |page=e0166409 |pmid=27893775 |pmc=5125594 |bibcode=2016PLoSO..1166409D }}</ref> Introduction of aquatic ] is widespread. It modifies ecosystems, causing biodiversity loss, and can harm fisheries. Harmful species include fish but are not limited to them;<ref name="Lovell Stone 2006">{{cite journal |last1=Lovell |first1=Sabrina J. |last2=Stone |first2=Susan F. |last3=Fernandez |first3=Linda |title=The economic impacts of aquatic invasive species: a review of the literature |journal=Agricultural and Resource Economics Review |volume=35 |issue=1 |year=2006 |pages=195–208 |doi=10.1017/S1068280500010157}}</ref> the arrival of a ] in the Black Sea damaged the ] fishery there.<ref>{{cite book |last1=Knowler |first1=D. |last2=Barbier |first2=E.B. |chapter=The Economics of an Invading Species: a Theoretical Model and Case Study Application |title=The Economics of Biological Invasions |editor1=Perrings, C. |editor2=Williamson, M. |editor3=Dalmazzone, S. |year=2000 |publisher=Edward Elgar |location=Cheltenham |pages=70–93}}</ref><ref name="Lovell Stone 2006"/> The opening of the ] in 1869 made possible ], facilitating the arrival of hundreds of Indo-Pacific marine species of fish, algae and invertebrates in the ], deeply impacting its overall biodiversity <ref>Atlas of Exotic Fishes in the Mediterranean Sea. 2nd Edition. 2021. (F. Briand Ed.) CIESM Publishers, Paris, Monaco 366 p.</ref> and ecology.<ref name="Golani 1998">{{cite journal |last=Golani |first=Daniel |title=Impact of Red Sea fish migrants through the Suez Canal on the aquatic environment of the Eastern Mediterranean |journal=Bulletin Series Yale School of Forestry and Environmental Studies |issue=103 |year=1998 |pages=375–387}}</ref> The predatory ] was deliberately introduced to ] in the 1960s as a commercial and sports fish. The lake had high biodiversity, with some 500 ] species of ] fish. It drastically altered the lake's ecology, and ] from multi-species to just three: the Nile perch, the ], and another introduced fish, the ]. The ] cichlid populations have collapsed.<ref name="Coulter Allanson Bruton 1986">{{cite journal |last1=Coulter |first1=George W. |last2=Allanson |first2=Brian R. |last3=Bruton |first3=Michael N. |last4=Greenwood |first4=P. Humphry |last5=Hart |first5=Robert C. |last6=Jackson |first6=Peter B. N. |last7=Ribbink |first7=Anthony J. |title=Unique qualities and special problems of the African Great Lakes |journal=Environmental Biology of Fishes |publisher=] |volume=17 |issue=3 |year=1986 |issn=0378-1909 |doi=10.1007/bf00698196 |pages=161–183|bibcode=1986EnvBF..17..161C }}</ref><ref name="Achieng 1990">{{cite journal |last=Achieng |first=A. P. |year=1990 |title=The impact of the introduction of the Nile Perch, ''Lates niloticus'' (L.), on the fisheries of Lake Victoria |journal=] |volume=37, Suppl. A |pages=17–23 |doi=10.1111/j.1095-8649.1990.tb05016.x |bibcode=1990JFBio..37S..17A }}</ref>

== Importance to humans ==

=== Economic ===

{{main|Commercial fishing|Fish farming}}

] hauling in a large catch of ], 2016 ]]

Throughout history, humans have used ] for ]. Historically and today, most fish harvested for human consumption has come by means of catching wild fish. However, fish farming, which has been practiced since about 3,500 BCE in ancient China,<ref>{{cite magazine |last1=Spalding |first1=Mark |title=Sustainable Ancient Aquaculture |url=http://voices.nationalgeographic.com/2013/07/11/sustainable-ancient-aquaculture/ |magazine=National Geographic |access-date=13 August 2015 |date=11 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150518210243/http://voices.nationalgeographic.com/2013/07/11/sustainable-ancient-aquaculture/ |archive-date=18 May 2015}}</ref> is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish.<ref name="Helfman">{{cite book |last1=Helfman |first1=Gene S. |title=Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources |date=2007 |publisher=] |isbn=978-1-59726-760-1 |pages=11}}</ref> ] is accordingly a large global business which provides income for millions of people.<ref name="Helfman"/> The ] has a guide on which fish are safe to eat, given the state of pollution in today's world, and which fish are obtained in a sustainable way.<ref name="EDF fish">{{cite web |url=https://seafood.edf.org/ |title=EDF Seafood Selector: Fish Choices that are Good for You and the Oceans |publisher=] |access-date=21 January 2024}}</ref> As of 2020, over 65 million tonnes (Mt) of marine fish and 10 Mt of freshwater fish were captured, while some 50 Mt of fish, mainly freshwater, were farmed. Of the marine species captured in 2020, ] represented 4.9 Mt, ] 3.5 Mt, ] 2.8 Mt, and ] and ] 1.6 Mt each; eight more species had catches over 1 Mt.<ref>{{cite book |year=2022 |title=The State of World Fisheries and Aquaculture 2022. Towards Blue Transformation |location=Rome |publisher=] |doi=10.4060/cc0461en |hdl=10535/3776 |isbn=978-92-5-136364-5 |url=https://doi.org/10.4060/cc0461en}}</ref>

=== Recreation ===

{{further|Fishkeeping|Recreational fishing}}

Fish have been recognized as a source of beauty for almost as long as used for food, appearing in ], being raised as ] in ponds, and displayed in ]s in homes, offices, or public settings. Recreational fishing is fishing primarily for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit, or ], which is fishing primarily for food. The most common form of recreational fishing employs a ], ], ], ], and a wide range of ]. Recreational fishing is particularly popular in North America and Europe; government agencies often actively manage target fish species.<ref>{{cite book |title=The Angler in the Environment: Social, Economic, Biological, and Ethical Dimensions |publisher=] |year=2011 |isbn=978-1-934874-24-0 |editor-last=Beard |editor-first=T. Douglas|location=Bethesda, Maryland |pages=365}}</ref><ref>{{cite book |title=Recreational Fisheries: Social, Economic and Management Aspects |publisher=] |year=1998 |isbn=978-0-852-38248-6 |editor-last=Hickley |editor-first=Phil |pages=328 |editor-last2=Tompkins |editor-first2=Helena}}</ref>

=== Culture ===

{{main|Fish in culture}}

Fish themes have symbolic significance in many religions. In ancient ], fish offerings were made to the gods from the very earliest times.<ref name="BlackGreen1992">{{cite book |last1=Black |first1=Jeremy |first2=Anthony |last2=Green |title=Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary |url=https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana |publisher=] |year=1992 |isbn=978-0-7141-1705-8 |pages=82–83 |url-status=live |archive-url=https://web.archive.org/web/20180220073403/https://books.google.com/books?id=05LXAAAAMAAJ&q=Inana |archive-date=20 February 2018}}</ref> Fish were also a major symbol of ], the god of water.<ref name="BlackGreen1992"/> Fish frequently appear as filling motifs in ]s from the ] ({{circa}} 1830 BC – {{circa}} 1531 BC) and ] (911–609 BC) periods.<ref name="BlackGreen1992"/> Starting during the ] ({{circa}} 1600 BC – {{circa}} 1155 BC) and lasting until the early ] (550–30 BC), healers and exorcists dressed in ritual garb resembling the bodies of fish.<ref name="BlackGreen1992"/> During the ] (312–63 BC), the legendary Babylonian ] ] was said to have dressed in the skin of a fish.<ref name="BlackGreen1992"/> Fish were sacred to the Syrian goddess ]<ref name="Hyde2008">{{cite book |last=Hyde |first=Walter Woodburn |date=2008 |orig-year=1946 |title=Paganism to Christianity in the Roman Empire |url=https://books.google.com/books?id=H_VLAwAAQBAJ&q=Atargatis |location=Eugene, Oregon |publisher=] |isbn=978-1-60608-349-9 |pages=57–58 |access-date=12 December 2020 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010624/https://books.google.com/books?id=H_VLAwAAQBAJ&q=Atargatis |url-status=live}}</ref> and, during her festivals, only her priests were permitted to eat them.<ref name="Hyde2008"/> In the ], the central figure, a ] named ], is swallowed by a giant fish after being thrown overboard by the crew of the ship he is travelling on.<ref name="Sherwood">{{cite book |last=Sherwood |first=Yvonne |date=2000 |title=A Biblical Text and Its Afterlives: The Survival of Jonah in Western Culture |url=https://books.google.com/books?id=Kivijc6ZX5QC&q=Jonah |publisher=] |isbn=978-0-521-79561-6 |pages=1–8 |access-date=12 December 2020 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010619/https://books.google.com/books?id=Kivijc6ZX5QC&q=Jonah |url-status=live}}</ref> ] used the '']'', a symbol of a fish, to represent Jesus.<ref name="Hyde2008"/><ref name="Coffman2008">{{cite magazine |url=http://www.christianitytoday.com/ch/asktheexpert/oct26.html |first=Elesha |last=Coffman |title=What is the origin of the Christian fish symbol? |magazine=] |date=8 August 2008 |access-date=13 August 2015 |archive-date=30 January 2016 |archive-url=https://web.archive.org/web/20160130092733/http://www.christianitytoday.com/ch/asktheexpert/oct26.html |url-status=live }}</ref> Among the ] said to take the form of a fish are ] of the ],<ref name="TeAra">{{cite web| url=https://teara.govt.nz/en/ngarara-reptiles/page-2| title='Ngārara – reptiles, Page 2. From sea to land', Te Ara - the Encyclopedia of New Zealand| publisher=Bradford Haami | accessdate=4 May 2018}}</ref>
the shark-god ] of ],<ref>{{cite book |last=Thrum |first=Thomas |title=Hawaiian Folk Tales |url=https://www.sacred-texts.com/pac/hft/hft27.htm |publisher=] |year=1907 |page=86}}</ref>
and ] of the Hindus.<ref>{{Cite book |last=Bandyopadhyaya |first=Jayantanuja |url=https://books.google.com/books?id=gwUF11NRyT4C&dq=matsya+avatar+Purana&pg=PA136 |title=Class and Religion in Ancient India |date=2007 |publisher=Anthem Press |isbn=978-1-84331-332-8 |page=136 |access-date=10 July 2022 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008120302/https://books.google.com/books?id=gwUF11NRyT4C&dq=matsya+avatar+Purana&pg=PA136 |url-status=live }}</ref> The constellation ] ("The Fishes") is associated with a legend from Ancient Rome that ] and her son ] were rescued by two fishes.<ref>] '']'' 2.457ff</ref>

Fish feature prominently in art,<ref name=Moyle>{{cite journal |last1=Moyle |first1=Peter B. |author1link=Peter B. Moyle |last2=Moyle |first2=Marilyn A. |title=Introduction to fish imagery in art |journal=Environmental Biology of Fishes |date=May 1991 |volume=31 |issue=1 |pages=5–23 |doi=10.1007/bf00002153 |bibcode=1991EnvBF..31....5M |s2cid=33458630 }}</ref> in films such as '']''<ref>{{cite journal |last=Tidwell |first=Christy |title='Fish Are Just like People, Only Flakier': Environmental Practice and Theory in ''Finding Nemo'' |journal=Americana: The Journal of American Popular Culture |issue=8 |year=2009 |url=https://www.americanpopularculture.com/journal/articles/spring_2009/tidwell.htm}}</ref> and books such as '']''.<ref>{{cite journal |last1=Durga |first1=P. |last2=Sai |first2=Kanaka |title=Nature of Existential Struggle in ''The Old Man and the Sea'' |journal=Journal of English Language and Literature JOELL |volume=4 |issue=4 |year=2017 |pages=19–21}}</ref> Large fish, particularly sharks, have frequently been the subject of ] and ], notably the novel '']'', made into a film which in turn has been parodied and imitated many times.<ref>{{cite book |last1=Alabaster |first1=Jay |editor1-last=Jackson |editor1-first=Kathy Merlock |editor2-last=Simpson |editor2-first=Philip L. |title=This shark, swallow you whole": Essays on the Cultural Influence of Jaws |date=2023 |publisher=] |isbn=978-1-4766-7745-3 |pages=124– |chapter=The Goofy Great White: Jaws and Our Love for an Apex Predator}}</ref> Piranhas are shown in a similar light to sharks in films such as '']''.<ref>{{cite web |last1=Zollinger |first1=Sue Anne |title=Piranha – Ferocious Fighter or Scavenging Softie? |url=http://indianapublicmedia.org/amomentofscience/piranhaferocious-fighter-scavenging-softie/ |website=A Moment of Science |publisher=] |access-date=1 November 2015 |date=3 July 2009 |url-status=live |archive-url=https://web.archive.org/web/20151017010303/http://indianapublicmedia.org/amomentofscience/piranhaferocious-fighter-scavenging-softie/ |archive-date=17 October 2015}}</ref>

<gallery mode=packed widths=150 heights=180>
File:Matsya painting.jpg|] of ] as a ], India
File:Bartolomeo Passerotti - The Fishmonger's Shop - WGA17072.jpg|''The Fishmonger's Shop'', ], 1580s
File:Goldfish Matisse.jpg|'']'' by ], 1912
</gallery>


== See also == == See also ==
{{main|Outline of fish}}
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* ] – bone used for determining the age of a fish
* ]
* ]
* ] (Bone used for determining the age of a fish)
* ]
* ] * ]
}}
{{refend}}

== Notes ==

{{notelist}}


== References == == References ==
{{refs|2}}


{{reflist}}
==External links==

{{wiktionary}}
== Sources ==
{{commonscat|Actinopterygii}}
* {{cite book |last=Benton |first=M. J. |date=2005 |url=https://books.google.com/books?id=VThUUUtM8A4C |title=Vertebrate Palaeontology |publisher=] |edition=3rd |isbn=978-140514449-0 }}
{{commonscat|Freshwater aquarium fishes}}
* {{cite book |last1=Helfman |first1=G. |last2=Collette |first2=B. |last3=Facey |first3=D. |title=The Diversity of Fishes |publisher=] |year=1997 |isbn=978-0-86542-256-8 |edition=1st}}
{{commonscat|Marine aquarium fishes}}
* {{cite book |last=Nelson |first=Joseph S. |chapter=Taxonomic Diversity |title=Fishes of the World |year=2016 |publisher=] |isbn=978-1-118-34233-6}}
{{commonscat|Fishes}}

* - Fisheries and Aquaculture Department: Fish and seafood utilization
== Further reading ==
* - Comprehensive database with information on over 29,000 fish species
* {{cite web |ref=none |last1=Eschmeyer |first1=William N. |last2=Fong |first2=Jon David |year=2013 |url=http://researcharchive.calacademy.org/research/ichthyology/ |title=Catalog of Fishes |publisher=] |access-date=28 February 2013 |archive-date=21 November 2018 |archive-url=https://web.archive.org/web/20181121203659/http://researcharchive.calacademy.org/research/ichthyology/ |url-status=live}}
* - Illustrated database of freshwater fishes of Australia and New Guinea
* {{cite book |ref=none |last1=Helfman |first1=G. |last2=Collette |first2=B. |last3=Facey |first3=D. |last4=Bowen |first4=B. |year=2009 |url=http://www.blackwellpublishing.com/helfman/ |title=The Diversity of Fishes: Biology, Evolution, and Ecology |publisher=] |isbn=978-1-4051-2494-2 |edition=2nd |access-date=26 January 2010 |archive-date=26 August 2021 |archive-url=https://web.archive.org/web/20210826212914/http://www.blackwellpublishing.com/helfman/ |url-status=live}}
* - Photos and facts on freshwater fishes of Southeast Asia
* Moyle, Peter B. (1993) {{Webarchive |url=https://web.archive.org/web/20230317010703/https://books.google.com/books?id=cu-J9tqy4IQC&q=Fish%3A+An+Enthusiast%27s+Guide |date=17 March 2023 }} University of California Press. {{ISBN |978-0-520-91665-4}} – good lay text.
* - Illustrated database of the freshwater fishes of Germany (in German)
* {{cite book |ref=none |last1=Moyle |first1=Peter B. |last2=Cech |first2=Joseph J. |year=2003 |title=Fishes, An Introduction to Ichthyology |edition=5th |publisher=Benjamin Cummings |isbn=978-0-13-100847-2}}
* - Conservation and study of North American freshwater fishes
* {{cite book |ref=none |author-link=Helen Scales |last=Scales |first=Helen |year=2018 |title=Eye of the shoal: A Fishwatcher's Guide to Life, the Ocean and Everything |publisher=Bloomsbury Sigma |isbn=978-1-4729-3684-4}}
* - Digital collection of freshwater and marine fish images
* {{cite book |ref=none |author-link=Neil Shubin |last=Shubin |first=Neil |year=2009 |url=https://books.google.com/books?id=c008kdNwR1cC |title=Your inner fish: A journey into the 3.5 billion year history of the human body |publisher=] |isbn=978-0-307-27745-9 |access-date=15 December 2015 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317010712/https://books.google.com/books?id=c008kdNwR1cC |url-status=live}} {{Webarchive |url=https://web.archive.org/web/20210114070358/https://books.google.com/books?id=c008kdNwR1cC |date=14 January 2021 }}
* - Child-oriented edutainment at the Liverpool Museum

* - Database with thousands of Philippine Fishes photographed in natural habitat
== External links ==
{{Commons category multi|Fish|Actinopterygii|align=right|Marine aquarium fish|Freshwater aquarium fish}}
{{Wikiquote}}
* – Illustrated database of freshwater fishes of Australia and New Guinea
* – Comprehensive database with information on over 29,000 fish species
* {{Webarchive |url=https://archive.today/20121215023321/http://fishdata.siu.edu/ |date=15 December 2012 |title=Fisheries and Illinois Aquaculture Center – Data outlet for fisheries and aquaculture research center in the central US}}
* {{Webarchive |url=https://web.archive.org/web/20080312223952/http://www.nativefish.org/ |date=12 March 2008 |title=The Native Fish Conservancy – Conservation and study of North American freshwater fishes}}
* – Fisheries and Aquaculture Department: Fish and seafood utilization

{{diversity of fish|state=expanded}}
{{Aquatic ecosystems}}
{{Marine aquatic ecosystems}}
{{Evolution of fish}}
{{Toxins}}
{{Brood parasite}}
{{Taxonbar|from=Q152}}
{{Authority control}}


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Latest revision as of 00:44, 7 January 2025

Gill-bearing non-tetrapod aquatic vertebrates

For fish as eaten by humans, see Fish as food. For the superclass of living fish, see Osteichthyes. For other uses, see Fish (disambiguation).

FishTemporal range: 535–0 Ma PreꞒ O S D C P T J K Pg N Middle CambrianRecent
Bala shark, a bony fish
Bala shark, a bony fish
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Olfactores
Subphylum: Vertebrata
Groups included
Jawless fish
Armoured fish
Spiny sharks
Cartilaginous fish
Bony fish
Ray-finned fish
Lobe-finned fish
Cladistically included but traditionally excluded taxa
Tetrapods

A fish (pl.: fish or fishes) is an aquatic, anamniotic, gill-bearing vertebrate animal with swimming fins and a hard skull, but lacking limbs with digits. Fish can be grouped into the more basal jawless fish and the more common jawed fish, the latter including all living cartilaginous and bony fish, as well as the extinct placoderms and acanthodians. In a break to the long tradition of grouping all fish into a single class (Pisces), contemporary phylogenetics views fish as a paraphyletic group.

Most fish are cold-blooded, their body temperature varying with the surrounding water, though some large active swimmers like white shark and tuna can hold a higher core temperature. Many fish can communicate acoustically with each other, such as during courtship displays. The study of fish is known as ichthyology.

The earliest fish appeared during the Cambrian as small filter feeders; they continued to evolve through the Paleozoic, diversifying into many forms. The earliest fish with dedicated respiratory gills and paired fins, the ostracoderms, had heavy bony plates that served as protective exoskeletons against invertebrate predators. The first fish with jaws, the placoderms, appeared in the Silurian and greatly diversified during the Devonian, the "Age of Fishes".

Bony fish, distinguished by the presence of swim bladders and later ossified endoskeletons, emerged as the dominant group of fish after the end-Devonian extinction wiped out the apex predators, the placoderms. Bony fish are further divided into the lobe-finned and ray-finned fish. About 96% of all living fish species today are teleosts, a crown group of ray-finned fish that can protrude their jaws. The tetrapods, a mostly terrestrial clade of vertebrates that have dominated the top trophic levels in both aquatic and terrestrial ecosystems since the Late Paleozoic, evolved from lobe-finned fish during the Carboniferous, developing air-breathing lungs homologous to swim bladders. Despite the cladistic lineage, tetrapods are usually not considered to be fish.

Fish have been an important natural resource for humans since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or in breeding cages in the ocean. Fish are caught for recreation, or raised by fishkeepers as ornaments for private and public exhibition in aquaria and garden ponds. Fish have had a role in human culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Etymology

The word fish is inherited from Proto-Germanic, and is related to German Fisch, the Latin piscis and Old Irish īasc, though the exact root is unknown; some authorities reconstruct a Proto-Indo-European root *peysk-, attested only in Italic, Celtic, and Germanic.

Evolution

Main article: Evolution of fish

Fossil history

Further information: Timeline of fish evolution
Dunkleosteus was a giant Devonian armoured placoderm, c. 400 mya.

About 530 million years ago during the Cambrian explosion, fishlike animals with a notochord and eyes at the front of the body, such as Haikouichthys, appear in the fossil record. During the late Cambrian, other jawless forms such as conodonts appear.

Jawed vertebrates appear in the Silurian, with giant armoured placoderms such as Dunkleosteus. Jawed fish, too, appeared during the Silurian: the cartilaginous Chondrichthyes and the bony Osteichthyes.

During the Devonian, fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes".

Phylogeny

Fishes are a paraphyletic group, since any clade containing all fish, such as the Gnathostomata or (for bony fish) Osteichthyes, also contains the clade of tetrapods (four-limbed vertebrates, mostly terrestrial), which are usually not considered fish. Some tetrapods, such as cetaceans and ichthyosaurs, have secondarily acquired a fish-like body shape through convergent evolution. Fishes of the World comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods". The biodiversity of extant fish is unevenly distributed among the various groups; teleosts, bony fishes able to protrude their jaws, make up 96% of fish species. The cladogram shows the evolutionary relationships of all groups of living fishes (with their respective diversity) and the tetrapods. Extinct groups are marked with a dagger (†); groups of uncertain placement are labelled with a question mark (?) and dashed lines (- - - - -).

Vertebrates

Jawless fishes (118 species: hagfish, lampreys)

?

Thelodonti, †Conodonta, †Anaspida

Galeaspida

Osteostraci

Jawed

Placodermi

?

Acanthodii

Chondrichthyes

 (>1,100 species: sharks, rays, chimaeras)

Osteichthyes
Lobe-finned fish
Actinistia

 (2 species: coelacanths)

Rhipidistia

Dipnoi (6 species: lungfish)

Tetrapoda (>38,000 species, not considered fish: amphibians, reptiles, birds, mammals)

Ray-finned fish
Cladistia

 (14 species: bichirs, reedfish)

Actinopteri
Chondrostei

 (27 species: sturgeons, paddlefish)

Neopterygii
Holostei

Ginglymodi (7 species: gars, alligator gars)

Halecomorphi (2 species: bowfin, eyetail bowfin)

Teleostei

 (>32,000 species)

vertebrates

Taxonomy

Main article: Taxonomy of fish

Fishes (without tetrapods) are a paraphyletic group and for this reason, the class Pisces seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three extant classes (Agnatha, Chondrichthyes, and Osteichthyes), and with extinct forms sometimes classified within those groups, sometimes as their own classes.

Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within the nine largest families; from largest to smallest, these are Cyprinidae, Gobiidae, Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families are monotypic, containing only one species.

Diversity

Main article: Diversity of fish

Fish range in size from the huge 16-metre (52 ft) whale shark to some tiny teleosts only 8-millimetre (0.3 in) long, such as the cyprinid Paedocypris progenetica and the stout infantfish.

Swimming performance varies from fish such as tuna, salmon, and jacks that can cover 10–20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second.

  • Fastest: e.g. salmon, 10–20 body lengths/second Fastest: e.g. salmon, 10–20 body lengths/second
  • Slowest: e.g. eel, 0.5 body lengths/second Slowest: e.g. eel, 0.5 body lengths/second

A typical fish is cold-blooded, has a streamlined body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and a tail fin, jaws, skin covered with scales, and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes.

Ecology

Habitats

Different fish species are adapted to a wide variety of freshwater and marine habitats.

Fish species are roughly divided equally between freshwater and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species. Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo, and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth.

Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., cusk-eels and snailfish), although none have been found in the deepest 25% of the ocean. The deepest living fish in the ocean so far found is a cusk-eel, Abyssobrotula galatheae, recorded at the bottom of the Puerto Rico Trench at 8,370 m (27,460 ft).

In terms of temperature, Jonah's icefish live in cold waters of the Southern Ocean, including under the Filchner–Ronne Ice Shelf at a latitude of 79°S, while desert pupfish live in desert springs, streams, and marshes, sometimes highly saline, with water temperatures as high as 36 C.

A few fish live mostly on land or lay their eggs on land near water. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows. A single undescribed species of Phreatobius has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged leaf litter. Cavefish of multiple families live in underground lakes, underground rivers or aquifers.

Parasites and predators

Further information: Fish diseases and parasites and Predatory fish

Like other animals, fish suffer from parasitism. Some species use cleaner fish to remove external parasites. The best known of these are the bluestreak cleaner wrasses of coral reefs in the Indian and Pacific oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract the attention of the cleaners. Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus, the cleaner, and the much larger E. suratensis.

Fish occupy many trophic levels in freshwater and marine food webs. Fish at the higher levels are predatory, and a substantial part of their prey consists of other fish. In addition, mammals such as dolphins and seals feed on fish, alongside birds such as gannets and cormorants.

Anatomy and physiology

Main articles: Fish anatomy and Fish physiology

Locomotion

Main article: Fish locomotion

The body of a typical fish is adapted for efficient swimming by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as control surfaces like an aircraft's flaps, enabling the fish to steer in any direction.

Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that allows them to adjust their buoyancy by increasing or decreasing the amount of gas it contains.

The scales of fish provide protection from predators at the cost of adding stiffness and weight. Fish scales are often highly reflective; this silvering provides camouflage in the open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility.

Circulation

The fish heart pumps blood to the gills, where it picks up oxygen. The blood then flows without further pumping to the body, from where it returns to the heart.

Fish have a closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through the gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart.

Respiration

Gills

Main article: Fish gill

Fish exchange gases using gills on either side of the pharynx. Gills consist of comblike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in the gills flows in the opposite direction to the water, resulting in efficient countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath a protective bony cover or operculum. They are able to oxygenate their gills using muscles in the head.

Air breathing

Further information: amphibious fish

Some 400 species of fish in 50 families can breathe air, enabling them to live in oxygen-poor water or to emerge on to land. The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to the heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of rock pools and the intertidal zone, are facultative air breathers, able to breathe air when out of water, as may occur daily at low tide, and to use their gills when in water. Some coastal fish like rockskippers and mudskippers choose to leave the water to feed in habitats temporarily exposed to the air. Some catfish absorb air through their digestive tracts.

Digestion

The digestive system consists of a tube, the gut, leading from the mouth to the anus. The mouth of most fishes contains teeth to grip prey, bite off or scrape plant material, or crush the food. An esophagus carries food to the stomach where it may be stored and partially digested. A sphincter, the pylorus, releases food to the intestine at intervals. Many fish have finger-shaped pouches, pyloric caeca, around the pylorus, of doubtful function. The pancreas secretes enzymes into the intestine to digest the food; other enzymes are secreted directly by the intestine itself. The liver produces bile which helps to break up fat into an emulsion which can be absorbed in the intestine.

Excretion

Most fish release their nitrogenous wastes as ammonia. This may be excreted through the gills or filtered by the kidneys. Salt is excreted by the rectal gland. Saltwater fish tend to lose water by osmosis; their kidneys return water to the body, and produce a concentrated urine. The reverse happens in freshwater fish: they tend to gain water osmotically, and produce a dilute urine. Some fish have kidneys able to operate in both freshwater and saltwater.

Brain

Diagram showing the pairs of olfactory, telencephalon, and optic lobes, followed by the cerebellum and the mylencephalon
Diagram of rainbow trout brain, from above

Fish have small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, notably mormyrids and sharks, which have brains about as large for their body weight as birds and marsupials. At the front of the brain are the olfactory lobes, a pair of structures that receive and process signals from the nostrils via the two olfactory nerves. Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the telencephalon, which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to the midbrain is the diencephalon; it works with hormones and homeostasis. The pineal body is just above the diencephalon; it detects light, maintains circadian rhythms, and controls color changes. The midbrain contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids. The hindbrain controls swimming and balance.The single-lobed cerebellum is the biggest part of the brain; it is small in hagfish and lampreys, but very large in mormyrids, processing their electrical sense. The brain stem or myelencephalon controls some muscles and body organs, and governs respiration and osmoregulation.

Sensory systems

Main article: Sensory systems in fish

The lateral line system is a network of sensors in the skin which detects gentle currents and vibrations, and senses the motion of nearby fish, whether predators or prey. This can be considered both a sense of touch and of hearing. Blind cave fish navigate almost entirely through the sensations from their lateral line system. Some fish, such as catfish and sharks, have the ampullae of Lorenzini, electroreceptors that detect weak electric currents on the order of millivolt.

Vision is an important sensory system in fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rods and cones (for scotopic and photopic vision); many species have colour vision, often with three types of cone. Teleosts can see polarized light; some such as cyprinids have a fourth type of cone that detects ultraviolet. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots.

Hearing too is an important sensory system in fish. Fish sense sound using their lateral lines and otoliths in their ears, inside their heads. Some can detect sound through the swim bladder.

Some fish, including salmon, are capable of magnetoreception; when the axis of a magnetic field is changed around a circular tank of young fish, they reorient themselves in line with the field. The mechanism of fish magnetoreception remains unknown; experiments in birds imply a quantum radical pair mechanism.

Cognition

Further information: Fish intelligence

The cognitive capacities of fish include self-awareness, as seen in mirror tests. Manta rays and wrasses placed in front of a mirror repeatedly check whether their reflection's behavior mimics their body movement. Choerodon wrasse, archerfish, and Atlantic cod can solve problems and invent tools. The monogamous cichlid Amatitlania siquia exhibits pessimistic behavior when prevented from being with its partner. Fish orient themselves using landmarks; they may use mental maps based on multiple landmarks. Fish are able to learn to traverse mazes, showing that they possess spatial memory and visual discrimination. Behavioral research suggests that fish are sentient, capable of experiencing pain.

Electrogenesis

The elephantnose fish is a weakly electric fish which generates an electric field with its electric organ and then uses its electroreceptive organs to locate objects by the distortions they cause in its electric field.
Further information: Electroreception and electrogenesis

Electric fish such as elephantfishes, the African knifefish, and electric eels have some of their muscles adapted to generate electric fields. They use the field to locate and identify objects such as prey in the waters around them, which may be turbid or dark. Strongly electric fish like the electric eel can in addition use their electric organs to generate shocks powerful enough to stun their prey.

Endothermy

Most fish are exclusively cold-blooded or ectothermic. However, the Scombroidei are warm-blooded (endothermic), including the billfishes and tunas. The opah, a lampriform, uses whole-body endothermy, generating heat with its swimming muscles to warm its body while countercurrent exchange minimizes heat loss. Among the cartilaginous fishes, sharks of the families Lamnidae (such as the great white shark) and Alopiidae (thresher sharks) are endothermic. The degree of endothermy varies from the billfishes, which warm only their eyes and brain, to the bluefin tuna and the porbeagle shark, which maintain body temperatures more than 20 °C (68 °F) above the ambient water.

Reproduction and life-cycle

Main article: Fish reproduction
Salmon fry hatching from the egg, keeping its yolk sac

The primary reproductive organs are paired testicles and ovaries. Eggs are released from the ovary to the oviducts. Over 97% of fish, including salmon and goldfish, are oviparous, meaning that the eggs are shed into the water and develop outside the mother's body. The eggs are usually fertilized outside the mother's body, with the male and female fish shedding their gametes into the surrounding water. In a few oviparous fish, such as the skates, fertilization is internal: the male uses an intromittent organ to deliver sperm into the female's genital opening of the female. Marine fish release large numbers of small eggs into the open water column. Newly hatched young of oviparous fish are planktonic larvae. They have a large yolk sac and do not resemble juvenile or adult fish. The larval period in oviparous fish is usually only some weeks, and larvae rapidly grow and change in structure to become juveniles. During this transition, larvae must switch from their yolk sac to feeding on zooplankton prey. Some fish such as surf-perches, splitfins, and lemon sharks are viviparous or live-bearing, meaning that the mother retains the eggs and nourishes the embryos via a structure analogous to the placenta to connect the mother's blood supply with the embryo's.

DNA repair

Embryos of externally fertilized fish species are directly exposed during their development to environmental conditions that may damage their DNA, such as pollutants, UV light and reactive oxygen species. To deal with such DNA damages, a variety of different DNA repair pathways are employed by fish embryos during their development. In recent years zebrafish have become a useful model for assessing environmental pollutants that might be genotoxic, i.e. cause DNA damage.

Defenses against disease

Further information: Immune system

Fish have both non-specific and immune defenses against disease. Non-specific defenses include the skin and scales, as well as the mucus layer secreted by the epidermis that traps and inhibits the growth of microorganisms. If pathogens breach these defenses, the innate immune system can mount an inflammatory response that increases blood flow to the infected region and delivers white blood cells that attempt to destroy pathogens, non-specifically. Specific defenses respond to particular antigens, such as proteins on the surfaces of pathogenic bacteria, recognised by the adaptive immune system. Immune systems evolved in deuterostomes as shown in the cladogram.

Deuterostomes

  Echinoderms, hemichordates, cephalochordates, urochordates

  Vertebrates

  Jawless fishes

  VLR adaptive immunity  

  Jawed fishes and tetrapods

 V(D)J adaptive immunity  
  innate immunity  

Immune organs vary by type of fish. The jawless fish have lymphoid tissue within the anterior kidney, and granulocytes in the gut. They have their own type of adaptive immune system; it makes use of variable lymphocyte receptors (VLR) to generate immunity to a wide range of antigens, The result is much like that of jawed fishes and tetrapods, but it may have evolved separately. All jawed fishes have an adaptive immune system with B and T lymphocytes bearing immunoglobulins and T cell receptors respectively. This makes use of Variable–Diversity–Joining rearrangement (V(D)J) to create immunity to a wide range of antigens. This system evolved once and is basal to the jawed vertebrate clade. Cartilaginous fish have three specialized organs that contain immune system cells: the epigonal organs around the gonads, Leydig's organ within the esophagus, and a spiral valve in their intestine, while their thymus and spleen have similar functions to those of the same organs in the immune systems of tetrapods. Teleosts have lymphocytes in the thymus, and other immune cells in the spleen and other organs.

Behavior

Shoaling and schooling

Main article: Shoaling and schooling
Fish such as these snipefishes school for safety from predators, and to spawn.

A shoal is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. A school is a much more tightly organised group, synchronising its swimming so that all fish move at the same speed and in the same direction. Schooling is sometimes an antipredator adaptation, offering improved vigilance against predators. It is often more efficient to gather food by working as a group, and individual fish optimise their strategies by choosing to join or leave a shoal. When a predator has been noticed, prey fish respond defensively, resulting in collective shoal behaviours such as synchronised movements. Responses do not consist only of attempting to hide or flee; antipredator tactics include for example scattering and reassembling. Fish also aggregate in shoals to spawn. The capelin migrates annually in large schools between its feeding areas and its spawning grounds.

Communication

See also: Acoustic communication in aquatic animals

Fish communicate by transmitting acoustic signals (sounds) to each other. This is most often in the context of feeding, aggression or courtship. The sounds emitted vary with the species and stimulus involved. Fish can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.

French grunt fish makes sounds by grinding its teeth.

Some fish produce sounds by rubbing or grinding their bones together. These sounds are stridulatory. In Haemulon flavolineatum, the French grunt fish, as it produces a grunting noise by grinding its teeth together, especially when in distress. The grunts are at a frequency of around 700 Hz, and last approximately 47 milliseconds. The longsnout seahorse, Hippocampus reidi produces two categories of sounds, 'clicks' and 'growls', by rubbing their coronet bone across the grooved section of their neurocranium. Clicks are produced during courtship and feeding, and the frequencies of clicks were within the range of 50 Hz-800 Hz. The frequencies are at the higher end of the range during spawning, when the female and male fishes were less than fifteen centimeters apart. Growls are produced when the H. reidi are stressed. The 'growl' sounds consist of a series of sound pulses and are emitted simultaneously with body vibrations.

Some fish species create noise by engaging specialized muscles that contract and cause swimbladder vibrations. Oyster toadfish produce loud grunts by contracting sonic muscles along the sides of the swim bladder. Female and male toadfishes emit short-duration grunts, often as a fright response. In addition to short-duration grunts, male toadfishes produce "boat whistle calls". These calls are longer in duration, lower in frequency, and are primarily used to attract mates. The various sounds have frequency range of 140 Hz to 260 Hz. The frequencies of the calls depend on the rate at which the sonic muscles contract.

The red drum, Sciaenops ocellatus, produces drumming sounds by vibrating its swimbladder. Vibrations are caused by the rapid contraction of sonic muscles that surround the dorsal aspect of the swimbladder. These vibrations result in repeated sounds with frequencies from 100 to >200 Hz. S. ocellatus produces different calls depending on the stimuli involved, such as courtship or a predator's attack. Females do not produce sounds, and lack sound-producing (sonic) muscles.

Conservation

The 2024 IUCN Red List names 2,168 fish species that are endangered or critically endangered. Included are species such as Atlantic cod, Devil's Hole pupfish, coelacanths, and great white sharks. Because fish live underwater they are more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the Devil's Hole pupfish occupies only a single 3 by 6 metres (10 by 20 ft) pool.

Overfishing

Main article: Overfishing
Collapse of the Atlantic northwest cod fishery

The Food and Agriculture Organization reports that "in 2017, 34 percent of the fish stocks of the world's marine fisheries were classified as overfished". Overfishing is a major threat to edible fish such as cod and tuna. Overfishing eventually causes fish stocks to collapse, because the survivors cannot produce enough young to replace those removed. Such commercial extinction does not mean that the species is extinct, merely that it can no longer sustain a fishery. In the case of the Pacific sardine fishery off the California coast, the catch steadily declined from a 1937 peak of 800,000 tonnes to an economically inviable 24,000 tonnes in 1968. In the case of the Atlantic northwest cod fishery, overfishing reduced the fish population to 1% of its historical level by 1992. Fisheries scientists and the fishing industry have sharply differing views on the resiliency of fisheries to intensive fishing. In many coastal regions the fishing industry is a major employer, so governments are predisposed to support it. On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be destroyed within fifty years.

Other threats

A key stress on both freshwater and marine ecosystems is habitat degradation including water pollution, the building of dams, removal of water for use by humans, and the introduction of exotic species including predators. Freshwater fish, especially if endemic to a region (occurring nowhere else), may be threatened with extinction for all these reasons, as is the case for three of Spain's ten endemic freshwater fishes. River dams, especially major schemes like the Kariba Dam (Zambezi river) and the Aswan Dam (River Nile) on rivers with economically important fisheries, have caused large reductions in fish catch. Industrial bottom trawling can damage seabed habitats, as has occurred on the Georges Bank in the North Atlantic. Introduction of aquatic invasive species is widespread. It modifies ecosystems, causing biodiversity loss, and can harm fisheries. Harmful species include fish but are not limited to them; the arrival of a comb jelly in the Black Sea damaged the anchovy fishery there. The opening of the Suez Canal in 1869 made possible Lessepsian migration, facilitating the arrival of hundreds of Indo-Pacific marine species of fish, algae and invertebrates in the Mediterranean Sea, deeply impacting its overall biodiversity and ecology. The predatory Nile perch was deliberately introduced to Lake Victoria in the 1960s as a commercial and sports fish. The lake had high biodiversity, with some 500 endemic species of cichlid fish. It drastically altered the lake's ecology, and simplified the fishery from multi-species to just three: the Nile perch, the silver cyprinid, and another introduced fish, the Nile tilapia. The haplochromine cichlid populations have collapsed.

Importance to humans

Economic

Main articles: Commercial fishing and Fish farming
A trawler hauling in a large catch of cod, 2016

Throughout history, humans have used fish as a food source for dietary protein. Historically and today, most fish harvested for human consumption has come by means of catching wild fish. However, fish farming, which has been practiced since about 3,500 BCE in ancient China, is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish. Fishing is accordingly a large global business which provides income for millions of people. The Environmental Defense Fund has a guide on which fish are safe to eat, given the state of pollution in today's world, and which fish are obtained in a sustainable way. As of 2020, over 65 million tonnes (Mt) of marine fish and 10 Mt of freshwater fish were captured, while some 50 Mt of fish, mainly freshwater, were farmed. Of the marine species captured in 2020, anchoveta represented 4.9 Mt, Alaska pollock 3.5 Mt, skipjack tuna 2.8 Mt, and Atlantic herring and yellowfin tuna 1.6 Mt each; eight more species had catches over 1 Mt.

Recreation

Further information: Fishkeeping and Recreational fishing

Fish have been recognized as a source of beauty for almost as long as used for food, appearing in cave art, being raised as ornamental fish in ponds, and displayed in aquariums in homes, offices, or public settings. Recreational fishing is fishing primarily for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit, or artisanal fishing, which is fishing primarily for food. The most common form of recreational fishing employs a rod, reel, line, hooks, and a wide range of baits. Recreational fishing is particularly popular in North America and Europe; government agencies often actively manage target fish species.

Culture

Main article: Fish in culture

Fish themes have symbolic significance in many religions. In ancient Mesopotamia, fish offerings were made to the gods from the very earliest times. Fish were also a major symbol of Enki, the god of water. Fish frequently appear as filling motifs in cylinder seals from the Old Babylonian (c. 1830 BC – c. 1531 BC) and Neo-Assyrian (911–609 BC) periods. Starting during the Kassite Period (c. 1600 BC – c. 1155 BC) and lasting until the early Persian Period (550–30 BC), healers and exorcists dressed in ritual garb resembling the bodies of fish. During the Seleucid Period (312–63 BC), the legendary Babylonian culture hero Oannes was said to have dressed in the skin of a fish. Fish were sacred to the Syrian goddess Atargatis and, during her festivals, only her priests were permitted to eat them. In the Book of Jonah, the central figure, a prophet named Jonah, is swallowed by a giant fish after being thrown overboard by the crew of the ship he is travelling on. Early Christians used the ichthys, a symbol of a fish, to represent Jesus. Among the deities said to take the form of a fish are Ikatere of the Polynesians, the shark-god Kāmohoaliʻi of Hawaiʻi, and Matsya of the Hindus. The constellation Pisces ("The Fishes") is associated with a legend from Ancient Rome that Venus and her son Cupid were rescued by two fishes.

Fish feature prominently in art, in films such as Finding Nemo and books such as The Old Man and the Sea. Large fish, particularly sharks, have frequently been the subject of horror movies and thrillers, notably the novel Jaws, made into a film which in turn has been parodied and imitated many times. Piranhas are shown in a similar light to sharks in films such as Piranha.

See also

Main article: Outline of fish

Notes

  1. The temperature is often around 0 C. The freezing point of seawater at the surface is -1.85 C, falling to -2.62 C at a depth of 1000 metres. However, the water can be supercooled somewhat below these temperatures.

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