Poecilia is a genus of fishes in the family Poeciliidae Poecculent of the order Cyprinodontiformes. 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 and a population of P. mexicana lives in caves (other populations of this species are surface-living). Some common and widespread species are often kept as aquarium fish, while other have very small ranges and are seriously threatened. All species in Poecilia are called mollies except for the Endler's livebearer (P. wingei) and the well-known guppy (P. reticulata). Without a modifying adjective, molly usually refers to the species Poecilia sphenops.