Nematobrycon is a small genus of characins (tetras) native to fast-flowing forest streams in Colombia. Its two species are compact, deep-bodied fish, 1.5 in as adults, best known for the elongated central rays of the caudal fin that give the genus its name (from the Greek for 'thread') and for pronounced sexual dimorphism - males carry longer fins and a larger, often iridescent blue eye than females. The atlas covers both described species, Nematobrycon lacortei and N. palmeri.
N. palmeri, the emperor tetra, is a long-standing and popular community aquarium fish, prized for its violet-blue sheen, dark lateral stripe, and the male's trailing fins and eye-catching colour under the right lighting; it is generally easy to keep and breed in soft, slightly acidic water. N. lacortei, the rainbow tetra, is rarer in the hobby but shares the same body plan and striking iridescence. Both species are listed as Data Deficient by the IUCN, meaning their conservation status has not been properly assessed despite the emperor tetra's familiarity in aquariums.