Gambusia is the mosquitofish genus, a group of small livebearing fish in the family Poeciliidae. The atlas covers 44 species, all modest in size — from 1 in up to 3 in — sharing the poeciliid livebearer body plan: a flattened head, an upturned mouth suited to picking prey and insect larvae from the surface, and internal fertilisation via a modified anal fin (gonopodium) in males, with females giving birth to free-swimming young rather than laying eggs. Atlas records span Mexico, the United States, Cuba, Guatemala, Haiti, Belize, Jamaica, and the Bahamas, with many species confined to individual springs, sinkholes, or short stretches of river in Texas, northern Mexico, and the Caribbean.
The genus has an unusually split identity. Two species, the western mosquitofish (Gambusia affinis) and eastern mosquitofish (G. holbrooki), were deliberately introduced worldwide through the 20th century as a biological control for mosquito larvae and are now among the most widely distributed freshwater fish on Earth — and among the most notorious invasive species, blamed for displacing native small fish and amphibians wherever released. Set against that, most remaining Gambusia species are the opposite story: narrow-range spring endemics under severe pressure from groundwater pumping and habitat loss. The atlas's IUCN tally reflects that split, with 2 species already Extinct, 3 Critically Endangered, 4 Endangered, and 6 Vulnerable among the 44 covered, and 17 species carrying CARES conservation-priority status — one of the highest counts of any genus in the atlas.