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| {{otheruses}} | |||
| ] at the ]]] | |||
| '''Fish''' are ] ] that are typically ]; covered with ]s, and equipped with two sets of paired ]s and several unpaired fins. Fish are abundant in the sea and in freshwaters, with species being known from mountain streams (e.g., ] and ]) as well as in the deepest depths of the ocean (e.g., ] and ]). They are of tremendous importance as food for people around the world, being either collected from the wild (see ]) or being farmed in much the same way as cattle or chickens (see ]). Fish are also exploited for recreation, through ] and ], and fish are commonly exhibited in public ]. Through the ages, many cultures have featured fish in their legends and myths, from the "great fish" that swallowed ] the Prophet through to the ever-popular half-human, half-fish ] around which books and movies are still centered (e.g., ]). Fish have been used as symbols in many different ways, from the ] used by early Christians through to the allegorical use of a ] in ] novel, ]. | |||
| ==What is a fish?== | |||
| The term "fish" is most precisely used to describe any non-] ], i.e., an animal with a backbone but lacking four limbs (or having ancestors that had four limbs). Unlike groupings such as ] or ], fish are not a single ] but a ] collection of ] including ], ], ], ], ], ], ], and ]. <ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0865422567</ref> | |||
| A typical fish is ]; has a ] body that allows it to swim rapidly; extracts oxygen from the water using ]; has two sets of paired fins, one or two dorsal fins, an anal fin, and a tail fin; has jaws; has skin that is covered with ]; and lays eggs that are fertilised externally. | |||
| ], a close relative of the ]. Their leaf-like appendages enable them to blend in with floating seaweed]] | |||
| However, to each of these there are exceptions. ] and some species of sharks are ], and 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 0865422567</ref> Streamling and swimming performance varies from highly streamlined and rapid swimmers able to reach 10-20 body-lengths per second (such as tuna, salmon, and ] through to slow but more manoeuvrable species such as ] 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 0865422567</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, ] 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 0865422567</ref> Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as ], ], ], 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 ] and ], ], 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 0865422567</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 ] ''Phreatobius cisternarum'' 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 various fish groups taken together account for more than half of the known vertebrates. There are at least 24,600 known species of fish, of which over 23,000 are bony fish, with the remainder being about 850 ] and about 85 hagfishes and lampreys. <ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0865422567</ref> They range in size from the 16 m (51 ft) ] to a 8 mm (just over ¼ of an inch) long ]. | |||
| Many types of ]s commonly referred to as "fish" are not fish in the sense given above. These include ], ], ], and ]. Marine invertebrates that are consumed as food are commonly called ]. ] and ] have been called fish as well, although they are ]. This usage is no longer common in English. | |||
| == 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. | |||
| Fish are classified into the following major groups: | |||
| * ] | |||
| ** ] (]s) | |||
| * ] (early jawless fish) | |||
| * ] | |||
| * ] | |||
| * ] (early jawless fish) | |||
| ** ] | |||
| ** ] | |||
| ** ] | |||
| * ] (jawed vertebrates) | |||
| ** ] | |||
| ** ] (cartilaginous fish) | |||
| ** ] | |||
| ** ] (bony fish) | |||
| *** ] (ray-finned fish) | |||
| *** ] (lobe-finned fish) | |||
| **** ] (]s) | |||
| **** ] (]) | |||
| Some palaeontologists consider that ] are ]s, and so regard them as primitive fish. | |||
| For a fuller treatment of classification, see the ] article. | |||
| ==Fish 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>]] | |||
| === Digestive system === | |||
| The advent of jaws allowed fish 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 ceca 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. | |||
| === Respiratory system === | |||
| Most fish exchange gases by using ] that are located on either side of the ]. Gills are made up of threadlike structures called ]. 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 ] and ], 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 ]. Some fishes, such as ], have developed an adaptation known as a ] that allows them to survive in oxygen-poor areas or places where bodies of water constantly dry up. These species of fish possess specialized organs that serve as ]. A tube brings air containing oxygen to this organ by way of the fish's mouth. Some kinds of lungfish are so dependent on receiving oxygen from the air that they will suffocate if not allowed to reach the surface of the water. | |||
| === Circulatory system === | |||
| Fish have a ] with a ] that pumps the ] in a single loop throughout the body. The blood goes from the heart to ], from the gills to the rest of the body, and then back to the heart. In most fishes, 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. The sinus venosus is a thin-walled sac that collects blood from the fish'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. | |||
| ==Homeothermy== | |||
| Although most fish are exclusively aquatic and cold-blooded, 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. Also, certain species of fish maintain elevated body temperatures to varying degrees. Endothermic ] (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 ] – shortfin mako, long fin mako, white, porbeagle, and salmon shark – 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 °C 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 ]. | |||
| === Excretory system=== | |||
| As with many aquatic animals, most fishes release their nitrogenous wastes as ]. Some of the wastes ] through the gills into the surrounding water. Others are removed by the ], 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. | |||
| === Sensory and nervous system === | |||
| Fish have well-developed nervous systems that organize around a central brain, that is divided into different parts. The most anterior, or front, end of the brain are the olfactory bulbs, which are involved in the fish's sense of smell. Unlike most vertebrates, the cerebrum of the fish primarily processes the sense of smell rather than being responsible for all voluntary actions. The optic lobes process information from the eyes. The cerebellum coordinates body movements while the medulla oblongata controls the functions of internal organs. Most fishes possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least 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. In 2003, it was also found by Scottish scientists at Edinburgh University performing research on rainbow trout that fish experience pain.{{fact}} Some fishes, such as catfish and sharks, have organs that detect low levels electric current. Other fishes, like the electric eel, can produce their own electricity. | |||
| === Muscular system === | |||
| ===Fish locomotion=== | |||
| {{main|Fish locomotion}} | |||
| 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. | |||
| === Reproductive system === | |||
| {{See|Spawning}} | |||
| The eggs of fish are fertilized either externally or internally, depending on species. The female usually lays the eggs, and the embryos in the eggs develop and hatch outside her body. These kind of fish are called ] fish. Oviparous fish develop by obtaining food from the yolk in the egg. ], for example, are oviparous. | |||
| ] fish keep the eggs inside of the mother's body after internal fertilization. Each embryo develops in its own egg. The young are "born alive" like most mammals. | |||
| Some species of fish, such as various sharks, are ]. Viviparous fish allow their embryos to stay in the mother's body like ovoviviparous fish. However, the embryos of viviparous fish obtain needed substances from the mother's body, not through material in the egg. The young of viviparous species are also "born alive". | |||
| ] is a term used to describe fish who give birth to live young. The eggs are fertilized internally by a male through an organ called a gonopodium. The eggs are kept inside the female until they hatch. The female then releases the fry into the water. Livebearer fry do not have egg yolks and can swim by themselves in under 24 hours. The most common livebearing species are the ], ], ] (molly) and ]. | |||
| ===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> | |||
| ==Evolution== | |||
| The early fossil record on fish is not very clear. It appears it was not a successful enough 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 branching to include land ] such as ], ], and ]. | |||
| The formation of the hinged jaw appears to be what resulted in the later proliferation of fish because un-jawed fish left very few ancestors. ]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. | |||
| 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 kept the larval form into adulthood (as some sea squirts do today, see ]), although the reversal of this case is also possible. Candidates for early fish include ] such as ], ], and ]. | |||
| ==Fish disease== | |||
| {{main|Fish diseases}} | |||
| ==Economic importance== | |||
| {{main|Aquaculture}} | |||
| {{main|Fishing}} | |||
| {{main|Fish farming}} | |||
| ==Recreation== | |||
| {{main|Angling}} | |||
| {{main|Fishkeeping}} | |||
| {{main|Sport fishing}} | |||
| == "Fish" or "fishes", "school" or '"shoal"? == | |||
| Though often used interchangeably, these pairs of 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 0865422567</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. | |||
| The collective noun for a random assemblage of fish merely using some localised resource such as food or nesting sites is known as an '''''aggregation'''''. When fish come together in n 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 (see article on ], the term used to cover such behaviours in animals). <ref>Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 375, 1997, ISBN 0865422567</ref> | |||
| * Cichlids congregating at ] sites form an aggregation. | |||
| * Many minnows and characins form shoals. | |||
| * Classic examples of schooling fish are anchovies, herrings, and silversides. | |||
| It should be noted that while school and shoal have different meanings within biology, they are often treated as ] by non-specialists, with speakers of ] using "shoal" to describe any grouping of fish, while speakers of ] often using "school" just as loosely. | |||
| ==References== | |||
| <references/> | |||
| == See also == | |||
| {{wiktionary}} | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| ==External links== | |||
| ===Taxonomy=== | |||
| * - Comprehensive database with information on over 29,000 fish species | |||
| ===Biogeography=== | |||
| * - Illustrated database of freshwater fishes of Australia and New Guinea | |||
| * - Photos and facts on freshwater fishes of Southeast Asia | |||
| * - Illustrated database of the freshwater fishes of Germany (in German) | |||
| * - Conservation and study of North American freshwater fishes | |||
| === Ichthyology=== | |||
| * - Digital collection of freshwater and marine fish images | |||
| * - Child-oriented edutainment at the Liverpool Museum | |||
| ==Wikimedia media== | |||
| {{commonscat|Actinopterygii}} | |||
| {{commonscat|Freshwater aquarium fishes}} | |||
| {{commonscat|Marine aquarium fishes}} | |||
| {{commonscat|Fish}} | |||
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Revision as of 18:54, 8 December 2006
>( <)">