Poecilia is a genus of fishes in the family Poeciliidae of the order Cyprinodontiformes.[1] These livebearers are native to fresh, brackish and salt water in the Americas, and some species in the genus are euryhaline. A few have adapted to living in waters that contain high levels of toxic hydrogen sulfide (H
2S)[2] and a population of P. mexicana lives in caves (other populations of this species are surface-living).[3]
Some common and widespread species are often kept as aquarium fish, while other have very small ranges and are seriously threatened.[2] All species in Poecilia are called mollies except for the Endler's livebearer (P. wingei) and the well-known guppy (P. reticulata) which have a distinctly different body shape from the rest of the genus.
Franz Steindachner first described the species in 1863. Poecilia refers to the Greek word poikilos, which means "with a lot of colours". Common names include "shortfin molly" and "Atlantic molly."[5] The type specimen was found in Orizaba, Mexico.
Fish of this genus have extremely variable coloration and have been selectively bred to create many different varieties. The most commonly kept species are guppies (P. reticulata), mollies (P. sphenops or P. latipinna), and Endler's livebearers (P. wingei). Members of the genus readily hybridize with eachother and so most commercially offered fish are hybrids (with guppies having some Endler, and mollies being a mix of common and sailfin mollies).[6]
They are easy to sex as males have a prominent gonopodium, a modified anal fin used to inseminate females. They mature quickly and breed readily, with females giving birth to a dozen or more fry every month.[7] Poecilids are cannibalistic and will eat any of their fry that are unable to flee in time (with females often eating the fry they just gave birth to, especially in a cramped aquarium setting).
If Poecilia in the wider sense is used then the species would be divided up into subgenera as follows:[13]