Livebearers

Live-bearing aquarium fish, often simply called livebearers, are fish that retain the eggs inside the body and give birth to live, free-swimming young.

Common aquarium livebearers

Species of interest to aquarists are almost always members of the family Poeciliidae, most commonly guppies, mollies, platies, swordtails, Endler's livebearer, and mosquito fish.

Live-bearing aquarium fish, often simply called livebearers, are fish that retain the eggs inside the body and give birth to live, free-swimming young. Because the newborn fish are large compared to the fry of oviparous fish, they 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 beginners to fish breeding. In addition, being much larger makes them far less vulnerable to predation, and with sufficient cover, they can sometimes mature in a community tank.

Ovoviviparous and viviparous fish compared

Most of the Poeciliidae are ovoviviparous, 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 are viviparous, with the eggs receiving food from the maternal blood supply through structures analogous to the placenta of placental mammals.

Aberrant livebearers and mouthbrooders

Seahorses and pipefish can 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.

Many cichlids are mouthbrooders, with the female (or more rarely the male) incubating the eggs in the buccal cavity. Compared with other cichlids, these species produce fewer but bigger eggs, and when they emerge, the fry are better developed and have a higher survivability. Because the eggs are protected from the environment but do not absorb nutrients from the parent, this condition is analogous to, though not identical with, ovoviviparity.