Kryptolebias is a genus of small killifish found along mangrove coastlines of the tropical and subtropical Americas, with atlas records spanning from the southeastern United States and the Caribbean down through Venezuela, Colombia, and Brazil. They live in the brackish, oxygen-poor water typical of mangrove swamps — flooded crab burrows, waterlogged leaf litter, hollow fallen logs — habitats few other fish can tolerate. The atlas's seven species run a small 1–3 in, with the elongate body and dorsally set eyes typical of surface-dwelling killifish.
The genus is best known through Kryptolebias marmoratus, the mangrove rivulus, one of the very few vertebrates capable of self-fertilization: individuals are internally self-fertilizing hermaphrodites, producing genetically near-identical offspring without needing a mate, a trait that has made the species a research model well beyond the aquarium hobby. Several Kryptolebias, including K. marmoratus, can also survive out of water for extended periods, tucked in damp leaf litter or rotting wood while their gills and skin handle gas exchange. Conservation status leans favorable for most of the genus — four species rank Least Concern — but one Brazilian species in the group is ranked Critically Endangered, a reminder that even a genus this broadly distributed can carry point-endemic populations at real risk from coastal development.