
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
Animals make use of a variety of modes of reproduction to produce their young. Traditionally this variety was classified into three modes, oviparity (embryos in eggs), viviparity (young born live), and ovoviviparity (intermediate between the first ...
in fish, providing different amounts of
parental care
Parental care is a behavioural and evolutionary strategy adopted by some animals, involving a parental investment being made to the evolutionary fitness of offspring. Patterns of parental care are widespread and highly diverse across the animal k ...
. In
ovoviviparity
Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop inside eggs that r ...
, there is
internal fertilization
Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For inte ...
and the young are born live but there is no placental connection or significant trophic (feeding) interaction; the mother's body maintains gas exchange but the unborn young are nourished by egg yolk. There are two types of
viviparity
In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...
in fish. In
histotrophic viviparity, the zygotes develop in the female's
oviduct
The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...
s, 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
The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...
.
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 animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...
in
mammals
A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...
, 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
Fish reproductive organs include testicle, testes and ovary, ovaries. In most species, gonads are Bilateria, paired organs of similar size, which can be partially or totally fused. There may also be a range of secondary organs that increase rep ...
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(s) and parents:
* Ovuliparity:
Fertilization
Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...
of eggs is external; zygotes develop externally.
* Oviparity: Fertilization of eggs is internal; zygotes develop externally as eggs with large
vitellus
Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
.
*
Ovoviviparity
Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop inside eggs that r ...
: Fertilization is internal; zygotes are retained in the female (or male) but without major trophic (feeding) interactions between zygote and parents (there may be minor interactions, such as maintenance of water and oxygen levels). The embryos depend upon their yolk for survival.
There are two types of
viviparity
In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...
among fish.
*
Histotrophic ("tissue eating") viviparity: The zygotes develop in the female's
oviduct
The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...
s, but she provides no direct nutrition. The embryos survive by eating her eggs or their unborn siblings.
*Hemotrophic ("blood eating") viviparity: The zygotes are retained within the female and are provided with nutrients by her, often through some form of
placenta
The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...
.
Diagram
Ovoviviparous fish
Examples of ovoviviparous fish are many of the
squaliform
The Squaliformes are an order (biology), order of sharks that includes about 126 species in seven families.
Members of the order have two dorsal fins, which usually possess spines, they usually have a sharp head, no anal fin or nictitating me ...
sharks, which include
sand sharks,
mackerel shark
The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the g ...
s,
nurse shark
The nurse shark (''Ginglymostoma cirratum'') is an Elasmobranchii, elasmobranch fish in the family Ginglymostomatidae. The conservation status of the nurse shark is globally assessed as Vulnerable species, Vulnerable in the IUCN Red List, IUCN L ...
s,
requiem sharks,
dog sharks and
hammerheads, among others, and the lobe finned
coelacanth
Coelacanths ( ) are an ancient group of lobe-finned fish (Sarcopterygii) in the class Actinistia. As sarcopterygians, they are more closely related to lungfish and tetrapods (the terrestrial vertebrates including living amphibians, reptiles, bi ...
. Some species of rockfish (''
Sebastes'') and
sculpins (
Comephoridae) produce rather weak larvae with no egg membrane and are also, by definition, ovoviviparous.
Ovoviviparity occurs in most live-bearing bony fishes (
Poeciliidae
Poeciliidae are a Family (biology), family of freshwater ray-finned fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, Poecilia, molly, Platy (fish), platy, and Green swo ...
).
Viviparous fish
Viviparous fish include the families
Goodeidae
Goodeidae is a family of teleost fish endemic to Mexico and some areas of the United States. Many species are known as splitfins. This family contains about 50 species within 18 Genus, genera. The family is named after ichthyologist George Brow ...
,
Anablepidae, Jenynsiidae,
Poeciliidae
Poeciliidae are a Family (biology), family of freshwater ray-finned fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, Poecilia, molly, Platy (fish), platy, and Green swo ...
,
Embiotocidae and some sharks (some species of the
requiem sharks, Carcharinidae and the hammerheads,
Sphyrnidae, among others). 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
The viviparous brotulas form a family, the Bythitidae, of ophidiiform fishes. They are known as viviparous brotulas as they generally bear live young, although there are indications that some species (at least '' Didymothallus criniceps'') do n ...
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" (yolk feeding) occurs when the mother provisions the
oocyte
An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ger ...
with all the resources it needs prior to fertilization, so the egg is independent of the mother. Many members of the fish family
Poeciliidae
Poeciliidae are a Family (biology), family of freshwater ray-finned fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, Poecilia, molly, Platy (fish), platy, and Green swo ...
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 (
oophagy or egg eating) or by eating their unborn siblings (
intra-uterine cannibalism
Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is also well document ...
).
"Matrotrophy" (mother feeding) 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.
*
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
Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
in 2001.
*
d The spadenose shark has the most advanced form of
placenta
The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...
l
viviparity
In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...
known in fish, as measured by the complexity of the placental connection and the difference in weight between the
egg
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...
and the newborn young.
*
e The eelpout
suckles its young embryos while still within their mother's body, making it the only
fish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
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
''Poeciliopsis''
Members of the genus ''Poeciliopsis'' (amongst others) show variable reproductive life history adaptations. ''P. monacha'' can be considered to be lecithotrophic because the female does not really provide any resources for her offspring after fertilization. ''P. lucida'' shows an intermediate level of matrotrophy, meaning that to a certain extent, the offspring's metabolism can actually affect the mother's metabolism, allowing for increased nutrient exchange. ''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.
''P. elongata'', ''P. turneri'' and ''P. presidionis'' form another clade which could be considered an outgroup to the ''P. monacha'', ''P.lucida'', and ''P. prolifica'' clade. These three species are very highly matrotrophic – so much so that in 1947, C. L. Turner described the follicular cells of ''P. turneri'' as "pseudo-placenta, pseudo-chorion, and pseudo-allantois".
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
In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once i ...
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.
Elasmobranchs
The majority of
elasmobranch
Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...
s are viviparous and show a wide range of strategies to provide their offspring with nourishment and respiratory requirements. Some sharks simply retain their young in the dilated posterior segment of the oviduct. In its simplest form, the uterus does not provide any additional nutrients to the embryos. However, other elasmobranchs develop secretory
uterine villi that produce histotroph, a nutrient which supplements the yolk stores of the oocyte. Uterine secretions are perhaps most advanced in the stingrays. Following depletion of the yolk, the uterine lining hypertrophies into secretory appendages termed "trophonemata". The process by which the uterine secretions (also known as uterine milk or histotroph) are produced resembles that of breast milk in mammals. Furthermore, the milk is rich in protein and lipid. As the embryo grows, vascularisation of the trophonemata enlarges to form
sinusoid
A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic wave whose waveform (shape) is the trigonometric sine function. In mechanics, as a linear motion over time, this is '' simple harmonic motion''; as rotation, it correspond ...
s that project out to the surface to form a functional respiratory membrane. In
lamnoid sharks, following yolk use, the embryos develop teeth and eat eggs and siblings within the uterus. There is usually one fetus per uterus and it grows to enormous proportions of up to 1.3 m in length. In placental sharks, the yolk sac is not withdrawn to become incorporated into the abdominal wall. Rather, it lengthens to form an
umbilical cord
In Placentalia, placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord i ...
and the yolk sac becomes modified into a functional epitheliochorial placenta.
Male pregnancy

The male fishes of
seahorse
A seahorse (also written ''sea-horse'' and ''sea horse'') is any of 46 species of small marine Osteichthyes, bony fish in the genus ''Hippocampus''. The genus name comes from the Ancient Greek (), itself from () meaning "horse" and () meanin ...
s,
pipefishes,
weedy and
leafy sea dragons (
Syngnathidae) are unusual as the male, rather than the female, incubates the eggs before releasing live fry into the surrounding water. To achieve this, male seahorses protect eggs in a specialized brood pouch, male sea dragons attach their eggs to a specific area on their bodies, and male pipefish of different species may do either.
When a female's eggs reach maturity, she squirts them from a chamber in her trunk via her
ovipositor
The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typica ...
into his brood pouch or egg pouch, sometimes called a "marsupium". 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.
Seahorse
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. Male juveniles develop pouches when they are 5–7 months old. The male carries the eggs for 9 to 45 days until the seahorses emerge fully developed, but very small. The number born maybe as few as five for smaller species, or 2,500 for larger species. A male seahorse's body has large amounts of
prolactin
Prolactin (PRL), also known as lactotropin and mammotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secr ...
, the same hormone that governs milk production in pregnant mammals and although the male seahorse does not supply milk, his pouch provides oxygen as well as a controlled-environment.
When the fry are ready to be born, the male expels them with muscular contractions, sometimes while attaching himself to seaweed with his 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.
The table below shows the gestation period and number of young born for some selected seahorses.
Pipefish

Pipefish brood their offspring either on distinct region of its body or in a brood pouch. Brood pouches vary significantly among different species of pipefish, but all contain a small opening through which the female's eggs can be deposited. The location of the brood pouch can be along the entire underside of the pipefish or just at the base of the tail, as with seahorses.
Pipefish in the genus ''
Syngnathus
''Syngnathus'' is a genus of fish in the family Syngnathidae found in marine, brackish and sometimes fresh waters of the Atlantic, Indian and Pacific Ocean. Fossils of these species are found from the Oligocene to the Pleistocene. They are kno ...
'' have a brood pouch with a ventral seam that can completely cover all of their eggs when sealed. In males without these pouches, eggs adhere to a strip of soft skin on the ventral surface of their bodies that does not contain any exterior covering – a type of "skin brooding".
At least two species of pipefish, ''
Syngnathus fuscus'' and ''
Syngnathus floridae'', provide nutrients for their offspring.
See also
*
Fish reproduction
Fish reproductive organs include testicle, testes and ovary, ovaries. In most species, gonads are Bilateria, paired organs of similar size, which can be partially or totally fused. There may also be a range of secondary organs that increase rep ...
References
{{Diversity of fish
Fertility
Fish physiology
Fish reproduction
Live-bearing fish
Ovoviviparous fish
Viviparous fish
Articles containing video clips