Cyprinodon is the pupfish genus, a group of small, hardy killifish in the family Cyprinodontidae built around one striking ecological theme: extreme habitat specialization. The atlas covers 44 species, most in the 1–4 in range, and while the genus as a whole spans a wide geography — atlas records reach Mexico, the United States, the Bahamas, the Dominican Republic, Cuba, Haiti, Jamaica, and Belize — individual species are often restricted to a single spring, sinkhole, saline lake, or short stretch of coastal lagoon. Pupfish get their name from the stubby, deep-bodied shape and darting, playful swimming of breeding males, which in many species turn bright blue and display and chase over territories on the substrate. Their defining trait is tolerance: populations persist in water that is unusually hot, unusually salty, or unusually low in oxygen, conditions few other fish survive.
Aquarium interest in Cyprinodon is real but specialist — hobbyists who keep pupfish do so for their toughness, their striking breeding colours, and often the conservation story attached to a given species, rather than for community-tank appeal. That story is central to the genus: the atlas's IUCN tally includes 11 Endangered, 9 Vulnerable, 4 Critically Endangered, 3 Extinct in the Wild, and 1 outright Extinct species among the 44 covered, making Cyprinodon one of the most threatened genera in the atlas. The isolated-spring lifestyle that makes pupfish scientifically fascinating also makes them acutely vulnerable — groundwater pumping, spring capping, and introduced species have driven several North American populations to the edge or over it, and 12 species in the genus carry CARES conservation-priority status in the atlas.
The same isolation has made Cyprinodon a textbook case of rapid evolution: on San Salvador Island in the Bahamas, a hypersaline lake holds a small radiation of pupfish that includes the scale-eating pupfish (Cyprinodon desquamator) alongside more generalist relatives, one of the fastest documented cases of a new feeding ecology evolving in a vertebrate. The best-known single species is probably the Devil's Hole pupfish, confined to a single flooded cavern in Nevada and long treated as a flagship case for endangered-species conservation in the United States.