Poecilia is a genus of livebearing fish in the family Poeciliidae, native to fresh and brackish waters across the Neotropics and into the southern United States, with the atlas's 58 species drawn from Brazil, Mexico, Guatemala, the Dominican Republic, Haiti, Honduras, Venezuela and Panama — a range that includes both mainland river systems and, through the group formerly separated as Limia, the freshwater habitats of Hispaniola and other Caribbean islands. Sizes span an unusually wide range for one genus, 0.5–6 in, from diminutive island endemics up to the larger mollies. Like all poeciliids, Poecilia fertilizes internally and gives birth to free-swimming young rather than laying eggs, with no parental care beyond the female carrying the brood to term.
Few fish genera matter more to the aquarium hobby: Poecilia includes the guppy (P. reticulata), arguably the single most widely kept aquarium fish on Earth and a long-standing subject of genetics and evolutionary-biology research, alongside the sailfin molly (P. latipinna) and shortfin molly (P. mexicana), both hardy staples that tolerate — and often prefer — some added salinity. Guppies and mollies are prolific, undemanding breeders that make them a common entry point into fishkeeping, but that same visibility masks real conservation concern elsewhere in the genus: the atlas records 3 Critically Endangered and 4 Endangered species, mostly narrow-range island or spring endemics, and lists 16 Poecilia species under its CARES program — a substantial share of the genus depending on hobbyist maintenance to buffer against habitat loss.