Poeciliidae - livebearers

Livebearers are viviparous, i.e. they bear their young live instead of laying eggs like most fishes. In males the anal fin is modified into a copulatory organ called a gonopodium. Males are smaller but more colorful than the females. The young of both sexes tend to have the coloration of the females. Many species are freshwater but can tolerate a wide variation in salinities. They are small fishes and are common in the aquarium trade.