Serrasalmus rhombeus (Redeye Piranha, and see below), is a fish of the piranha family Serrasalmidae found in South America in the Amazon and Orinoco river basins, the north and eastern Guiana Shield rivers, and northeastern Brazilian coastal rivers. Its length is up to 41.5 cm.
These piranhas live in very diverse habitats ranging from soft blackwater, to hard whitewater areas in the Amazon, and as such, are very tolerant of differing water chemistry.
This fish was long known and traded as Serrasalmus niger. It varies widely across its range and whether it really is a single species is still unknown, though in some cases, the fish in question are certainly mere morphs. Peruvian S. rhombeus are called jet black highbacks or Peruvian Black Piranhas. Brazilian Black Piranhas are actually greyish in color and some have diamond-shaped scales. Venezuelan S. rhombeus have the brightest red eyes and grey coloration.
Other names include Rhombeus Piranha, for its striking shape. Particularly light-colored populations are the White Piranhas of the aquarium trade, formerly believed to be the "true" S. rhombeus. A common local name is caribe ojo rojo; the species is also known as caribe amarillo, not to be confused with the other "yellow" piranhas S. gibbus and S. spilople.