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Pregnancy in fish is the period between the fertilization of eggs and giving birth to live young. Related terms are gestation period and being gravid.

Types of reproduction and pregnancy

Five modes of reproduction can be differentiated in fish based on relations between the zygote(s) and parents:

• Ovuliparity: fecundation (fertilisation) of eggs and delopment of the zygotes are external.
• Oviparity: fecundation is internal, the female lays zygotes as eggs with large vitellus.
• Ovoviviparity: this is oviparity with retention of zygotes in the female’s body or in the male’s body, but without major trophic interactions between zygote and parents (there may be minor effects, such as maintenance of water and oxygen levels). The embryos depend upon their yolk for survival. This occurs in most live-bearing bony fishes (Poeciliidae). In seahorses, zygotes are retained in the male’s ventral "marsupium".

Typically, viviparous fish have a structure analogous to the placenta seen in mammals connecting the mother's blood supply to that of the embryo. Examples of viviparous fish include surf-perches, splitfins and the lemon shark.

• Histotrophic viviparity: the zygotes develop in the female’s oviducts, but find their nutrients by oophagy or intra-uterine cannibalism. This occurs in some sharks.
• Hemotrophic viviparity: nutrients are provided by the female, often through some form of placenta.

Aquarists commonly refer to ovoviviparous and viviparous fish as livebearers. Examples include guppies, mollies, moonfish, platys, four-eyed fish and swordtails. All of these varieties exhibit signs of their pregnancy before the live fry are born. Common livebearing aquarium fish generally are pregnant for an average of 28 days, but this can vary between 21-40 days.

Comparison between species

The table below shows the gestation period and number of young born for some selected fish.

• The spadenose shark has the most advanced form of placental viviparity known in fish, as measured by the complexity of the placental connection and the difference in weight between the egg and the newborn young.
• The eelpout suckles its young embryos while still within their mother's body, making it the only fish species to suckle its offspring.
• Only one pregnant female is known to have been caught; she was carrying six unborn young.

Guppy

Guppies are highly prolific livebearers giving birth to between five and 30 fry, though under extreme circumstances, she may give birth to only one or two or over 100. The gestation period of a guppy is typically 21–30 days, but can vary considerably. The area where a pregnant guppy’s abdomen meets the tail is sometimes called the "gravid patch", or "gravid spot". When pregnant, there is a slight discoloration that slowly darkens as the guppy progresses through pregnancy. The patch first has a yellowish tinge, then brown and become deep orange as the pregnancy develops. This patch is where the fertilized eggs are stored and grow. The darkening is actually the eyes of the developing baby guppies and the orange tinge is their jelly-like eggs.

Seahorse

Seahorses are unusual as the male, rather than the female, incubates the eggs before releasing live fry into the surrounding water. When a female's eggs reach maturity, she squirts approximately 200 eggs from a chamber in her trunk into his pouch. Her body slims as his bulges. Both seahorses then sink back to the bottom and she swims off.

A male seahorse's body has large amounts of prolactin, the same hormone that governs milk production in pregnant women. He doesn't supply milk, but his pouch provides oxygen as well as a controlled-environmentfor about 21 days. The pregnant male typically gives birth during the night, and a female returning for the morning greeting finds her mate ready for the next batch of eggs.

Others

The striped seaperch or surfperch (Embiotoca lateralis) is a saltwater fish with a gestation period of forty weeks, or ten months. This gives the family its scientific name, from Greek "embios" meaning "persistent" and tokos meaning "birth".

References

1. Lodé, T. (2001). Les Stratégies de Reproduction des Animaux. Dunod Sciences, Paris.
2. "How long is fish pregnant before gives birth? Which fish is born alive and which makes eggs?". Retrieved October 15, 2014.
3. "Atlantic Sharpnose Shark". Florida Museum of Natural History. Retrieved October 16, 2014.
4. Compagno, L.J.V., M. Dando and Fowler, S. (2005). Sharks of the World. Princeton University Press. pp. 260–261. ISBN 978-0-691-12072-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
5. Van Der Elst, R. (1993). A Guide to the Common Sea Fishes of Southern Africa. Struik. p. 367. ISBN 9781868253944.
6. Compagno, L.J.V. (1984). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known To Date. Food and Agriculture Organization of the United Nations.
7. "Sphyrna tiburo". Fishbase.org. Retrieved October 16, 2014.
8. McAuley, R.B., Simpfendorfer, C.A., Hyndes, G.A. and Lenanton, R.C.J. (2007). "Distribution and reproductive biology of the sandbar shark, Carcharhinus plumbeus (Nardo), in Western Australian waters". Mar. Freshwater Res. 58 (1): 116–126. doi:10.1071/MF05234.{{cite journal}}: CS1 maint: multiple names: authors list (link)
9. "Rhizoprionodon porosus". Fishbase.com. Retrieved October 16, 2014.
10. ^ Fowler, S.L., R.D. Cavanagh, M. Camhi, G.H. Burgess, G.M. Cailliet, S.V. Fordham, C.A. Simpfendorfer, and J.A. Musick (2005). Sharks, Rays and Chimaeras: The Status of the Chondrichthyan Fishes. International Union for Conservation of Nature and Natural Resources. ISBN 2-8317-0700-5.{{cite book}}: CS1 maint: multiple names: authors list (link) Cite error: The named reference "fowler et al" was defined multiple times with different content (see the help page).
11. Feldheim, K. A.; Gruber, S. H.; Ashley, M. V. (22 August 2002). "The breeding biology of lemon sharks at a tropical nursery lagoon". Proceedings of the Royal Society B: Biological Sciences. 269 (1501): 1655–1661. doi:10.1098/rspb.2002.2051.
12. Scott, P. (1997). Livebearing fish. Tetra Press. ISBN 1-56465-193-2.
13. Template:IUCN2010
14. Baremore, I.E. and Hale, L.F. (2012). "Reproduction of the sandbar shark in the Western North Atlantic Ocean and Gulf of Mexico". Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science. 4. American Fisheries Society: 560–572. doi:10.1080/19425120.2012.700904.{{cite journal}}: CS1 maint: multiple names: authors list (link)
15. ^ Matt Walker (September 28, 2010). "Pregnant European eelpout fish suckles young embryos". BBC News. Retrieved October 22, 2014.
16. ^ The Shark Trust. "Basking Shark Factsheet". The Shark Trust. Retrieved 2006-07-07.
17. "Bat ray". Monterey Bay Aquarium Online Field Guide. Retrieved October 22, 2014.
18. Lavett Smith, C.; Rand, Charles S.; Schaeffer, Bobb; Atz, James W. (1975). "Latimeria, the living coelacanth, is ovoviviparous". Science. 190 (4219): 1105–6. Bibcode:1975Sci...190.1105L. doi:10.1126/science.190.4219.1105.
19. Wourms, J.P. (1993). "Maximization of evolutionary trends for placental viviparity in the spadenose shark, Scoliodon laticaudus". Environmental Biology of Fishes. 38: 269–294. doi:10.1007/BF00842922.
20. "Guppy". Encyclopædia Britannica Online. 2007. Archived from the original on 13 May 2008. Retrieved 7 May 2007. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
21. Milius, S. (2000). "Pregnant—and still macho" (PDF). Science New Online. Retrieved October 6, 2014.
22. "How long does a fish stay pregnant?". Retrieved October 15, 2014.

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