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. Among aquarium fish, livebearers are nearly all members of the Poeciliidae family and include guppies, mollies, platies and swordtails.

The advantages of livebearing to the aquarist are that the newborn juvenile fish are larger than newly-hatched fry, have a lower chance of mortality and are easier to care for. Unusual livebearers include seahorses and pipefish, where the males care for the young, and certain cichlids that are mouthbrooders, with the parent incubating the eggs in the buccal cavity.

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. Most of these are ovoviviparous, with the developing embryos receiving no nourishment from the parent fish, but a few are viviparous, receiving food from the maternal blood supply.

Goldfish are not livebearers as they lay eggs. The female lays the eggs and as she does, the male fertilizes them. To breed goldfish, the eggs must be removed from the parents as they will eat the eggs.

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