The Order Cyprinodontiformes contains the rivulines, killifishes and live bearers. The latter group occur in the Superfamily Peocilioidea; there are two Families, the Anablepidae (Four-eyed Fish and relatives) and the Poeciliidae (commonly, the Poeciliids). Rather recently two subfamilies were transferred from Cyprinodontidae to Poeciliidae, thus changing the livebearers to the subfamily rank of Poeciliinae. The name comes from the Greek poikilos, meaning "with different colours." Fishbase lists 293 species contained within 30 genera.
The Family Poeciliidae is known as livebearers, and aquarists tend to think of them as occurring chiefly in Mexico and Central America, but they actually extend from SE United States down into northern Argentina. The majority of species are either true live-bearing species [viviparous] or have internal fertilization with the eggs hatching before being laid so that the female produces live young [ovoviviparous]. Except for one genus, all the American species are live bearing. But there are also some egg scattering species with external fertilization, and these all occur in Africa. The distribution of the Subfamily in Africa and America suggests that the Poeciliidae pre-date the separation of Africa and South America that occurred during the early Cretaceous period some 130 million years ago. Live bearing subsequently evolved in the American species.
The degree to which the female supports the developing larvae in many species is lecithotrophic, which means the mother provides the oocyte (the female reproductive cell) with everything it needs prior to fertilization so that the egg develops independent of the female. Some species like the splitfins and halfbeaks are matrotrophic ["mother feeding"], which means the source of nourishment provided by the egg yolk is supplemented directly from the mother after fertilization.
Of interest to almost all aquarists are the genera Poecilia (molly, guppy and endler), Xiphophorus (platy and swordtail) and Gambusia, with Limia (21 species, found on the islands of the Greater Antilles) and Belonesox (pike livebearer) of interest more to advanced aquarists.
All livebearers require basic (alkaline) water and moderate hardness. They are generally hardy fish, and easy to breed which makes them popular with beginning aquarists. However, mollies especially are highly sensitive to ammonia, and should never be introduced to a tank that is not fully cycled.
The male's anal fin is modified into a gonopodium by which he transfers sperm into the female. This is the most reliable means of determining the sex of the fish; in some species the male is also the more colourful, and in the case of the swordtail the male possess the extended lower lobe of the caudal fin. If both sexes are to be housed in the same aquarium, there should be more females than males to provide some rest for the females from the attention of the males. Maintaining males and females in the same tank will result in regular batches of fry, and once impregnated, a female can deliver several successive batches on her own [this is known as superfetation]. In batches of fry, this impregnation can occur before the males attain their colouration, so fish must be separated out very early if the aquarist wishes to selectively breed them. A female can deliver fry roughly every 28 days. Fry require hiding places such as thick floating plant cover, or they will be readily eaten by the adults.
Fish readily interbreed within their respective genus, creating the variety of colourful forms especially among the platies and swordtails available today.