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, typically around 10 mm in length, they are easier to feed than newly hatched fry of egg-laying species such as characins and cichlids. This makes them much easier to raise. In addition, being much bigger they are also far less vulnerable to predation, and are often able to swim and hide effectively even in quite densely stocked community tanks. Livebearers are consequently among the very first fish aquarists are able to breed.

Common aquarium livebearers
The coelacanth, surf perches, and many types of sharks and rays are all livebearing fish, these are not usually kept by aquarists. Instead, the species of interest to aquarists are almost always members of the family Poeciliidae, most commonly guppies, mollies, platies and swordtails. A few other genera from this family are kept by advanced aquarists, including Ameca, Brachyrhaphis, Heterandria, and Limia.

Rare livebearers
A few other families include livebearing species kept by aquarists, though far less commonly than the Poeciliidae. These include the splitfins, family Goodeidae; the halfbeaks, family Hemirhamphidae; and the four-eyed fish Anableps anableps, family Anablepidae. 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.

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 being receive 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 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. Since the eggs are invariably large and fewer in number than is the case with species that lay their eggs in nests, once the young fish emerge they are larger and more highly developed than is typical for cichlids, and are relatively durable and easy to raise. The eggs do not absorb nutrients from the parent and the fry are not released until they are large and comparatively well developed. This condition is analogous to, though not identical with, ovoviviparity. A wide range of other fish are also known to be mouthbrooders, including arowanas and sea catfish, though these are less commonly kept in aquaria.


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