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 relatively 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.
A few other genera from Poeciliidae are kept by advanced aquarists, including Ameca, Brachyrhaphis, Heterandria, and Limia. Other livebearing species kept by aquarists, though far less commonly than the Poeciliidae are the splitfins, family Goodeidae; and the halfbeaks, family Hemirhamphidae; Freshwater stingrays of the family Potamotrygonidae are sometimes kept by advanced aquarists, and these are also livebearers.
None of these are considered to be easy aquarium fish, and some present as much of a challenge to the aquarist as egg-laying fish. Commonly, the problems include providing the right diet and conditions to trigger mating, and ensuring that the female does not miscarry the developing embryos too soon.
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.
Seahorses and pipefish can be defined as livebearers, although in these cases it is the males that 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.