The Pirapitinga (Piractus brachypomus), also called the red-bellied pacu, is a large species of pacu, a close relative of piranhas and silver dollars, in the serrasalmid family. It is native to the Amazon basin in tropical South America, but it formerly included populations in the Orinoco, only described in 2019 as a separate species, P. orinoquensis. Additionally, the pirapitinga has been introduced to other regions. In South Florida they are invasive in rivers, canals or lakes.
Juveniles have a distinct red chest and belly, and are easily confused with the carnivorous red-bellied piranha (Pygocentrus nattereri), but the two can be separated by their teeth, which are molar-like in Piaractus. This similarity is believed to be mimicry by the pirapitinga in an attempt of avoiding predation by other species. Adults lack the bright red chest and belly, and resemble the tambaqui (Colossoma macropomum), but can be separated by several meristic and morphological features.
The pirapitinga is migratory, but the pattern is poorly understood. Spawning occurs at the beginning of the flood season between November and February. Larvae of the pirapitinga are found in whitewater rivers, but adults mainly live in flooded forests and floodplains of variours river types, including both nutrient-rich and nutrient-poor.
It mainly feeds on fruits, seeds and nuts, but it is opportunistic and will also eat zooplankton, insects, crustaceans and small fish, especially in the dry season. In general, more seeds are able to pass undamaged through the pirapitinga than the tambaqui, meaning that the former is overall a more efficient seed disperser.