Livebearers

Livebearersarefishthat retain their eggs inside the body and give birth to live, free-swimming young. They are especially prized by aquarium owners. Among aquarium fish, livebearers are nearly all members of the familyPoeciliidaeand include:guppies,mollies,platiesandswordtails.^[1]

The advantages of livebearing to the aquarist are that the newbornjuvenile fishare larger than newly-hatched fry, have a lower chance of mortality and are easier to care for. Unusual livebearers includeseahorsesandpipefish,where the males care for the young, and certaincichlidsthat aremouthbrooders,with the parent incubating the eggs in thebuccal cavity.

Common aquarium livebearers

Species of interest to aquarists are almost always members of the familyPoeciliidae,most commonlyguppies,mollies,platies,swordtails,Endler's livebearer,andmosquitofish.Most of these areovoviviparous,with the developing embryos receiving no nourishment from the parent fish, but a few areviviparous,receiving food from the maternal blood supply.^{[citation needed]}

Because the newborn fish are large compared to the fry ofoviparousfish, which are those that lay eggs, newborn fish of livebearers are easier to feed than the fry of egg-laying species, such as characins and cichlids. This makes them much easier to raise, and for this reason, aquarists often recommend them for beginning fish breeder hobbyists. The larger livebearer fry makes them far less vulnerable to predation, as the parents often eat fry if hungry. With the sufficient cover in the way of plants or porous objects, they can sometimes mature in a community tank.^[2]

Ovoviviparous and viviparous fish compared

Most of the Poeciliidae areovoviviparous,that is, while the eggs are retained inside the body of the female for protection, the eggs are essentially independent of the mother and she does not provide them with any nutrients. In contrast, fish such as splitfins and halfbeaks areviviparous,with the eggs receiving food from the maternal blood supply through structures analogous to theplacentaofplacental mammals.^[3]

Aberrant livebearers and mouthbrooders

Seahorsesandpipefishcan be defined as livebearers, although in these cases the males incubate the eggs rather than the females. In many cases, the eggs are dependent on the male for oxygen and nutrition so these fish can be further defined as viviparous livebearers.^{[citation needed]}

Manycichlidsaremouthbrooders,with the female (or more rarely the male) incubating the eggs in the mouth. Compared with other cichlids, these species produce fewer but bigger eggs, and when they emerge, the fry is better developed and has higher survivability. Because the eggs are protected from the environment but do not absorb nutrients from the parent, this condition isanalogousto, though notidenticalwith, ovoviviparity.^{[citation needed]}