Pregnancy in fish
Pregnancy has been traditionally defined as the period of time eggs are incubated in the body after the egg-sperm union.
In histotrophic viviparity, the zygotes develop in the female's oviducts, but she provides no direct nutrition; the embryos survive by eating her eggs or their unborn siblings.
Some species of rockfish (Sebastes) and sculpins (Comephoridae) produce rather weak larvae with no egg membrane and are also, by definition, ovoviviparous.
[11] "Aplacental viviparity" occurs when the female retains the embryos during the entire time of development but without any transfer of nutrients to the young.
[12] Post-fertilization transfer of nutrients has been reported in several species within the genera Gambusia and Poecilia, specifically, G. affinis, G. clarkhubbsi, G. holbrooki, G. gaigei, G. geiseri, G. nobilis, P. formosa, P. latipinna, and P.
[citation needed] method (Days) (Average) Members of the genus Poeciliopsis (amongst others) show variable reproductive life history adaptations.
P. prolifica is considered to be highly matrotrophic, and almost all of the nutrients and materials needed for foetal development are supplied to the oocyte after it has been fertilized.
This level of matrotrophy allows Poeciliopsis to carry several broods at different stages of development, a phenomenon known as superfetation.
[citation needed] The majority of elasmobranchs are viviparous and show a wide range of strategies to provide their offspring with nourishment and respiratory requirements.
However, other elasmobranchs develop secretory uterine villi that produce histotroph, a nutrient which supplements the yolk stores of the oocyte.
During a mammalian pregnancy, the placenta allows the female to nourish her progeny in the womb, and remove their waste products.
If male pipefish and seahorses provide only a simple pouch for fish eggs to develop and hatch, it might not fully qualify as bona-fide pregnancy.
However, current research suggests that in syngnathid species with well developed brood pouches, males do provide nutrients, osmoregulation and oxygenation to the embryos they carry.
[44] When mating, the female seahorse deposits up to 1,500 (average of 100 to 1,000) eggs in the male's pouch, located on the ventral abdomen at the base of the tail.
Birth typically occurs during the night, and a female returning for the routine morning greeting finds her mate ready for the next batch of eggs.
Brood pouches vary significantly among different species of pipefish, but all contain a small opening through which the female's eggs can be deposited.
[49] Pipefish in the genus Syngnathus have a brood pouch with a ventral seam that can completely cover all of their eggs when sealed.
