Pygocentrus is the genus of "true" piranhas, deep-bodied predatory characins (family Serrasalmidae, the same family as the pacus) found across major South American river systems - the Amazon, Orinoco, Parana-Paraguay, and Sao Francisco basins, matching the wide country list in the atlas (Brazil, Venezuela, Argentina, Bolivia, Colombia, Guyana, Paraguay, Peru). The genus's defining trait is the tightly interlocking, single row of triangular, razor-edged teeth set in a short, powerful jaw, paired with a laterally compressed, deep body. Atlas species run 11–19.5 in, sizable fish by characin standards, and most shoal at least as juveniles, thinning out into smaller groups or solitary habits as adults.
The red-bellied piranha (P. nattereri) is the species most people picture when they hear the word piranha, and it is by far the most widely kept in the aquarium trade - a striking, silver-and-red predatory fish that draws specialist keepers willing to provide a large tank and accept its territorial temperament, though ownership is restricted or banned outright in a number of jurisdictions. P. piraya, endemic to the Sao Francisco basin, is the largest member of the genus and holds outsized cultural weight in Brazil as a native river fish. Wild reputation aside - piranhas are opportunistic omnivores and scavengers more often than the frenzied hunters of popular myth - conservation status across the genus stays reassuring, with three atlas species rated Least Concern and one Not Evaluated.