Piaractus brachypomus, the Red-Bellied Pacu, is a large species of pacu, a close relative of piranhas and silver dollars, in the serrasalmid family.[1] It is native to the Amazon basin in tropical South America, but it formerly included populations in the Orinoco, which was described in 2019 as a separate species, P. orinoquensis.[2] Additionally, P. brachypomus is widely farmed and has been introduced to other regions.[3] In South Florida they are invasive in rivers, canals or lakes.[4]
As with a number of other closely related species, P. brachypomus is often referred to as the red-bellied pacu in reference to the appearance of the juveniles. This has resulted in a great deal of confusion about the nature and needs of all the species involved, with the reputation and requirements of one frequently being wrongly attributed to the others.[5]
In general, its behavior resembles that of the closely related tambaqui (Colossoma macropomum). It is migratory, but the pattern is poorly understood.[6] Spawning occurs at the beginning of the flood season between November and February.[7] Larvae of the pirapitinga are found in whitewater rivers, but adults mainly live in flooded forests and floodplains of various river types, including those of both nutrient-rich and nutrient-poor.[6] Unlike the tambaqui, the pirapitinga also occurs in the headwaters of nutrient-poor rivers (not just in the lower sections).[6]
It mainly feeds on fruits, seeds, and nuts, but it is opportunistic and will also take zooplankton, insects, crustaceans and small fish, especially in the dry season.[3] [8] 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.[9]
Piaractus brachypomus can reach up to 88abbr=onNaNabbr=on in length and 25kg (55lb) in weight.
Juveniles have a distinct red chest and stomach, 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 brachypomus.[1] This similarity is believed to be Batesian mimicry by P. brachypomus in an attempt of avoiding predation by other species.[3] 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 has a smaller adipose fin that lacks rays, as well as differences in teeth and operculum.[1] [10] The pirapitinga also has a more rounded head profile (less elongated and pointed).[10] [11] The other member of its genus, P. mesopotamicus, can be distinguished by its smaller scale-size[12] and the higher number of lateral scales (more than 110).[1]
The pirapitinga supports major fisheries and based on a review by IBAMA, it was the 12th most caught fish by weight in the Brazilian Amazon in 1998 (just after the tambaqui).[6]
The pirapitinga is often kept in aquaculture. Hybrids between this species and the tambaqui have been produced in aquaculture.[13] It can also hybridize with P. orinoquensis, but the offspring appears to be sterile.[2]