Laimosemion is a genus of small killifish in the family Rivulidae, native to forest streams of northern South America — the atlas's records come from Brazil, Guyana, French Guiana, Peru, Suriname, and Ecuador. The genus was split out of the once much broader Rivulus as taxonomists worked out that 'Rivulus' in the old, wide sense actually covered several distinct lineages; Laimosemion now holds many of the non-annual, Guiana-shield and Amazonian species from that split. Unlike the seasonal 'annual' killifish that hatch, breed, and die within a single wet season in temporary pools, most Laimosemion are non-annual, living in permanent or semi-permanent streams and leaf litter rather than surviving the dry season only as drought-resistant eggs. The atlas covers 13 species, ranging from about 0.5–2.5 in.
Laimosemion species turn up in the killifish hobby but are kept less often than the flashier annual killies of Africa and South America, in part because non-annual species can't be shipped or stored as eggs in peat the way annual killifish are — live fish have to be kept and moved year-round. Most are quiet, small-mouthed insectivores best suited to soft, blackwater-style conditions with dense leaf litter or fine-leaved plants standing in for their native stream-margin cover. Conservation data for the genus are thin: roughly half of the atlas's species are not yet formally evaluated, which is typical for a recently split, taxonomically active genus of small, hard-to-survey stream fish.