Nematolebias is a small genus of annual killifish in the family Rivulidae, native to coastal lowlands around Rio de Janeiro, Brazil. Like other annual killifish, its species live in seasonal pools that dry up between rainy seasons; the adult fish die out completely, and the population survives the dry period as fertilised eggs buried in the mud, entering a period of diapause before hatching when the rains return and the pools refill. The atlas covers 2 species, Nematolebias papilliferus and N. whitei, both modest fish of roughly 3 in.
N. whitei, the Rio pearlfish, is a long-established favourite among specialist killifish keepers for its vivid orange-red body and bold pattern, and for the challenge of its peat-based annual breeding cycle - eggs are collected in damp peat moss, stored through a simulated dry season, then rewetted to trigger hatching. Both species in the genus face real conservation pressure: one is assessed as Critically Endangered and the other as Endangered, reflecting how vulnerable seasonal-pool habitats are to drainage and development near Rio de Janeiro, and both are flagged in the atlas's CARES conservation-priority list.