Red bellied pacu

The tambaqui is the heaviest characin in the Americas (the lighter Salminus can grow longer) and the second heaviest scaled freshwater fish in South America (after the arapaima). It can reach up to 1.1 m (3.6 ft) in total length and 44 kg (97 lb) in weight, but a more typical size is 0.7 m (2.3 ft). The largest caught by rod-and-reel and recognized by IGFA weighed 32.4 kg (71 lb). After the flood season, around 10% of a tambaqui's weight is the visceral fat reserves and at least another 5% is fat found in the head and muscles. It is similar in shape to the piranha and juveniles are sometimes confused with the carnivorous fish; the tambaqui is tall and laterally compressed with large eyes and a slightly arched back. Unlike more predatory species, the teeth of the tambaqui are molar-like, an adaption for crushing plant seeds and nuts. The lower half of its body is typically mainly blackish. The remaining is mainly gray, yellowish or olive, but the exact hue varies considerably and depends in part on habitat with individuals in blackwater being much darker than individuals from whitewater. The pelvic, anal and small pectoral fins are black. The tambaqui resembles the pirapitinga (Piaractus brachypomus), but the latter species has a more rounded head profile (less elongated and pointed) and a smaller adipose fin that lacks rays, as well as differences in teeth and operculum. Hybrids between the tambaqui and the similar Piaractus (both species) have been produced in aquaculture, and are occasionally seen in the wild. The hybrid offspring can be difficult to identify by appearance alone.