Hypopygus is a genus of weakly electric knifefish in the order Gymnotiformes, found in slow-flowing streams, flooded forest, and leaf-litter habitats across the Amazon and Orinoco basins - the atlas's 10 species draw on Brazil, Venezuela, Colombia, Guyana, Peru, French Guiana, Suriname, and Bolivia. Like other gymnotiforms, Hypopygus lacks a caudal fin, tapering instead to a slender point, and swims by undulating a long anal fin rather than flexing its body. Atlas species run a modest 1.5–6 in. The genus generates a continuous weak electric field used for navigating and sensing its surroundings in dark, structurally cluttered water rather than for defense or hunting, and it is largely nocturnal, spending daylight hours tucked into leaf litter and root tangles.
Hypopygus has essentially no presence in the aquarium trade - it is a genus known chiefly to ichthyologists and researchers studying electroreception, where South American gymnotiforms are a long-standing model system for how weak electric organ discharges are generated and used for communication. Conservation data are thin (mostly Least Concern and Data Deficient, with one Vulnerable listing), which fits a genus documented mainly through scattered river surveys rather than targeted assessment.