Pregnancy in fish

Pregnancy has been traditionally defined as the period of time eggs are incubated in the body after the egg-sperm union. Although the term often refers to placental mammals, it has also been used in the titles of many international, peer-reviewed, scientific articles on fish. Consistent with this definition, there are several modes of reproduction in fish, providing different amounts of parental care. In ovoviviparity, there is internal fertilization and the young are born live but there is no placental connection or significant trophic interaction; the mother's body maintains gas exchange but the unborn young are nourished by egg yolk. There are two types of viviparity in fish. 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. In hemotrophic viviparity, the zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta.
In seahorses and pipefish, it is the male that becomes pregnant.

Types of reproduction and pregnancy

Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg-sperm union. Despite strong similarities between viviparity in mammals, researchers have historically been reluctant to use the term "pregnancy" for non-mammals because of the highly developed form of viviparity in eutherians. Recent research into physiological, morphological and genetic changes associated with fish reproduction provide evidence that incubation in some species is a highly specialized form of reproduction similar to other forms of viviparity. Although the term "pregnancy" often refers to eutherian animals, it has also been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g.
Five modes of reproduction can be differentiated in fish based on relations between the zygote and parents:

Ovuliparity: Fertilization of eggs is external; zygotes develop externally.
Oviparity: Fertilization of eggs is internal; zygotes develop externally as eggs with large vitellus.
Ovoviviparity: Fertilization is internal; zygotes are retained in the female but without major trophic interactions between zygote and parents. The embryos depend upon their yolk for survival.

There are two types of viviparity among fish.

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.
Hemotrophic viviparity: The zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta.

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Ovoviviparous fish

Examples of ovoviviparous fish are many of the squaliform sharks, which include sand sharks, mackerel sharks, nurse sharks, requiem sharks, dog sharks and hammerheads, among others, and the lobe finned coelacanth. Some species of rockfish and sculpins produce rather weak larvae with no egg membrane and are also, by definition, ovoviviparous. Ovoviviparity occurs in most live-bearing bony fishes.

Viviparous fish

Viviparous fish include the families Goodeidae, Anablepidae, Jenynsiidae, Poeciliidae, Embiotocidae and some sharks. The halfbeaks, Hemiramphidae, are found in both marine and freshwaters and those species that are marine produce eggs with extended filaments that attach to floating or stationary debris, while those that are found in freshwater are viviparous with internal fertilization. The Bythitidae are also viviparous although one species, Dinematichthys ilucoeteoides, is reported to be ovoviviparous.
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. As examples, the female swordtail and guppy will both give birth to anywhere from 20 to 100 live young after a gestation period of four to six weeks, and mollies will produce a brood of 20 to 60 live young after a gestation of six to 10 weeks.

Nutrition during pregnancy

Other terms relating to pregnancy in fish relate to the differences in the mode and extent of support the female gives the developing offspring.
"Lecithotrophy" occurs when the mother provisions the oocyte with all the resources it needs prior to fertilization, so the egg is independent of the mother. Many members of the fish family Poeciliidae are considered to be lecithotrophic, however, research is increasingly showing that others are matrotrophic.
"Aplacental viviparity" occurs when the female retains the embryos during the entire time of development but without any transfer of nutrients to the young. The yolk sac is the only source of nutrients for the developing embryo. There are at least two exceptions to this; some sharks gain nourishment by eating unfertilized eggs produced by the mother or by eating their unborn siblings.
"Matrotrophy" occurs when the embryo exhausts its yolk supply early in gestation and the mother provides additional nutrition. 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. mexicana.
Viviparous fish have developed several ways of providing their offspring with nutrition. "Embryotrophic" or "histrotrophic" nutrition occurs by the production of nutritive fluid, uterine milk, by the uterine lining, which is absorbed directly by the developing embryo. "Hemotrophic" nutrition occurs through the passing of nutritive substances between blood vessels of the mother and embryo that are in close proximity, i.e. a placenta-like organ similar to that found in mammals.

Comparison between species

There is considerable variation between species in the length of pregnancy. At least one group of fish has been named after its pregnancy characteristics. The surfperch, genus Embiotoca, is a saltwater fish with a gestation period of three to six months. This lengthy period of pregnancy gives the family its scientific name from the Greek "embios" meaning "persistent" and "tokos" meaning "birth".
The table below shows the gestation period and number of young born for some selected fish.

Species		Reproduction method	Gestation period	Number of young
Atlantic sharpnose shark		Viviparous	300-330	4-6
Barbeled houndshark		Viviparous^a	>120	7
Blackspot shark		Viviparous^b	270	1-2
Blue shark		Viviparous	270-366	4-135
Bonnethead shark		Viviparous^c		4-12
Bull shark		Viviparous	366	4-10
Butterfly goodeid		Viviparous	55-60	6-30
Caribbean sharpnose shark		Viviparous		2-6
Daggernose shark		Viviparous	366	2-8
Lemon shark		Viviparous	366	18
Oceanic whitetip shark		Viviparous	366	1-15
Dwarf seahorse		Ovoviviparous	3-55	10
Sandbar shark		Viviparous	366	8
Spadenose shark		Viviparous^d	150-180	6-18
Viviparous eelpout		Ovoviviparous	180	30-400
Basking shark		Ovoviviparous	>366	unknown^f
Bat ray		Ovoviviparous	270-366	2-10
Coelacanth		Ovoviviparous	>366
Blue stingray		Ovoviviparous	270	1-5
Bluespotted stingray		Ovoviviparous	90-150	1-7
Carpet sharks		Ovoviviparous	180	30-40
Knifetooth sawfish		Ovoviviparous	150	6-23
Nurse shark	,	Ovoviviparous	150	21-29
Sailfin molly		Ovoviviparous	21-28	10-140
Salmon shark		Ovoviviparous	270	2-6
Sand tiger shark		Ovoviviparous	270-366	2^g
School shark		Ovoviviparous	366	28-38
Shortfin mako shark		Ovoviviparous	450-540	4-18
Spotted eagle ray		Ovoviviparous	366	4
Tiger shark		Ovoviviparous	430-480	10-80
Tawny nurse shark		Aplacental viviparity		1-2

^a Unlike any other shark, the yolk-sac placenta is globular or spherical.
^b At first, the embryos are sustained by a yolk sac, but later a placenta develops.
^c A bonnethead female produced a pup by parthenogenesis in 2001.
^d 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.
^e The eelpout suckles its young embryos while still within their mother's body, making it the only fish species to suckle its offspring.
^f Only one pregnant female is known to have been caught; she was carrying six unborn young.
^g 1 per uterine horn