Characidium is a genus of small, slender-bodied characins commonly called darters or darter characins, found across much of tropical and subtropical South America. The atlas covers 84 species, ranging from just over 1 in to well over 19.5 in in one exceptional case, with records from Brazil, Guyana, Colombia, Argentina, Bolivia, Venezuela, Peru, and Uruguay. True to the common name, most species are benthic: rather than swimming continuously like typical characins, they perch on the substrate using their pectoral and pelvic fins, darting short distances between resting points in a way that echoes unrelated darters and gobies.
Characidium species favour fast, well-oxygenated riffles and rocky or sandy stream beds, and many feed on small invertebrates picked from the current or leaf litter. They are almost never seen in the aquarium trade; interest in the genus is mostly ichthyological rather than ornamental, though the few species that do reach hobbyists, sold simply as darter characins, appeal to stream-biotope and nano-aquarium keepers for their unusual perching behaviour rather than bright colour.
Most species in the atlas are Not Evaluated by the IUCN, a common situation for small, recently described, or narrowly distributed stream fish that have not yet been formally assessed. Among the species that have been evaluated, several, including some restricted to single coastal Brazilian drainages, are considered Critically Endangered, consistent with the vulnerability of stream specialists whose entire range can sit inside one threatened watershed.