Pregnancy in fish

Source: Wikipedia, the free encyclopedia.

A pregnant Southern platyfish

Pregnancy has been traditionally defined as the period of time eggs are incubated in the body after the egg-sperm union.

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

Birth of guppy fry

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

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.[1] 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.[2][3][4][5]

Five modes of reproduction can be differentiated in fish based on relations between the zygote(s) and parents:[6][7]

  • 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 (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 among fish.

  • Histotrophic ("tissue eating") 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 ("blood eating") viviparity: The zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta.


Diagram

Fish reproduction types
Ovuliparity

Oviparity

Ovoviviparity

Viviparity
Histotrophic

Histotrophic

Hemotrophic

Hemotrophic

Ovoviviparous fish

Examples of ovoviviparous fish are many of the

Comephoridae) produce rather weak larvae with no egg membrane and are also, by definition, ovoviviparous.[8][9] Ovoviviparity occurs in most live-bearing bony fishes (Poeciliidae
).

Viviparous fish

Viviparous fish include the families

Bythitidae are also viviparous although one species, Dinematichthys ilucoeteoides, is reported to be ovoviviparous.[8]

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.[10]

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.

matrotrophic.[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. 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 (

intra-uterine cannibalism
).

"Matrotrophy" (mother feeding) occurs when the embryo exhausts its yolk supply early in gestation and the mother provides additional nutrition.[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. mexicana.[11]

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.[8]

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.[13] 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.[citation needed]

Species Reproduction

method

Gestation period

(Days)

Number of young

(Average)

Atlantic sharpnose shark[14] (Rhizoprionodon terraenovae) Viviparous 300-330 4-6
Barbeled houndshark[15] (Leptocharias smithii) Viviparousa >120 7
Blackspot shark[16] (Carcharhinus sealei) Viviparousb 270 1-2
Blue shark[17] (Prionace glauca) Viviparous 270-366 4-135
Bonnethead shark (Sphyrna tiburo) Viviparousc 4-12[18]
Bull shark[19] (Carcharhinus leucas) Viviparous 366 4-10
Butterfly goodeid[20] (
Ameca splendens
) 		Viviparous 55-60 6-30
Caribbean sharpnose shark (Rhizoprionodon porosus) Viviparous 2-6[21]
Daggernose shark[22] (Isogomphodon oxyrhynchus) Viviparous 366 2-8
Lemon shark[23] (Negaprion brevirostris) Viviparous 366 18 (max)
Oceanic whitetip shark[24] (Carcharhinus longimanus) Viviparous 366 1-15
Dwarf seahorse[25] (Hippocampus zosterae) Ovoviviparous 3-55 10
Sandbar shark[26] (Carcharhinus plumbeus) Viviparous 366 8
Spadenose shark[22] (Scoliodon laticaudus) Viviparousd 150-180 6-18
Viviparous eelpout[27] (Zoarces viviparus) Viviparouse 180 30-400
Basking shark[28] (Cetorhinus maximus) Ovoviviparous >366 unknownf
Bat ray[29] (Myliobatis californica) Ovoviviparous 270-366 2-10
Coelacanth (g. Latimeria) Ovoviviparous >366[30]
Blue stingray (Dasyatis chrysonota) Ovoviviparous 270 1-5
Bluespotted stingray[31]
 (Neotrygon kuhlii) Ovoviviparous 90-150 1-7
Carpet sharks (f. Ginglymostomatidae) Ovoviviparous 180 30-40
Knifetooth sawfish[32] (Anoxypristis cuspidata) Ovoviviparous 150 6-23
Nurse shark (
Ginglymostoma cirratum
), 		Ovoviviparous 150 21-29
Sailfin molly (Poecilia latipinna) Ovoviviparous 21-28 10-140
Salmon shark[33] (Lamna ditropis) Ovoviviparous 270 2-6
Sand tiger shark[34] (Carcharias taurus) Ovoviviparous 270-366 2g
School shark[35] (Galeorhinus galeus) Ovoviviparous 366 28-38
Shortfin mako shark[36] (Isurus oxyrinchus) Ovoviviparous 450-540 4-18
Spotted eagle ray[37] (Aetobatus narinari) Ovoviviparous 366 4
Tiger shark[38] (Galeocerdo cuvier) Ovoviviparous 430-480 10-80
Tawny nurse shark[39]: 195–199  (Nebrius ferrugineus) Aplacental viviparity 1-2

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.[42]

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".[citation needed]

Guppy

Guppies are highly prolific livebearers

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.[citation needed
]

Elasmobranchs

The majority of

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 and the yolk sac becomes modified into a functional epitheliochorial placenta.[9]

Male pregnancy

Pregnant male seahorse
Main article: Male pregnancy

The male fishes of

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 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.[44]

Seahorse

Main article: 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, 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.[45]

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

Species Reproduction

method

Gestation period

(Days)

Number of young
Big-belly seahorse[46] (Hippocampus abdominalis) Ovoviviparous 28 600-700
Lined seahorse[47] (Hippocampus erectus) Ovoviviparous 20-21 650 (max)
Long-snouted seahorse[48] (Hippocampus guttulatus) Ovoviviparous 21 581 (max)

Pipefish

Main article: Pipefish
The subcaudal pouch of the male black-striped 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.[49] Pipefish in the genus Syngnathus 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".[50]

At least two species of pipefish,

Syngnathus floridae, provide nutrients for their offspring.[51]

See also

References

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  17. ^ 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.
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