A piranha or piraña is a member of a family of omnivorous freshwater fish which live in South American rivers. In Venezuelan rivers, they are called caribes. They are known for their sharp teeth and a voracious appetite for meat.
Piranhas belong to the subfamily Serrasalminae, which also includes closely related herbivorous fish such as pacus. Traditionally, only the four genera Pristobrycon, Pygocentrus, Pygopristis and Serrasalmus are considered to be true piranhas, due to their specialized teeth. However, a recent analysis showed that, if the piranha group is to be monophyletic, it should be restricted to Serrasalmus, Pygocentrus and part of Pristobrycon, or expanded to include these taxa plus Pygopristis, Catoprion, and Pristobrycon striolatus. Pygopristis was found to be more closely related to Catopricornis than the other three piranha genera.
The total number of piranha species is unknown and new species continue to be described. In 1988, it was stated that fewer than a half of the approximately 60 nominal species of piranhas at the time were valid. More recently (in 2003), one author recognized a total of 38 or 39 species, although the validity of some taxa remains questionable.
Piranhas are found in the Amazon basin, in the Orinoco, in rivers of the Guyanas, in the Paraguay-Paraná, and the São Francisco River systems. Some species of piranha have broad geographic ranges, occurring in more than one of the major basins mentioned above, whereas others appear to have more limited distributions.
Aquarium piranhas have been introduced into parts of the United States with specimens occasionally found in the Potomac River and even as far north as Lake Winnebago in Wisconsin, although they typically do not survive cold winters. Piranhas have also been discovered in the Kaptai Lake in south-east Bangladesh. More recently a dead Piranha was discovered in a river in Devon, England. Research is being carried out to establish how piranha have moved to such distant corners of the world from their original habitat. It is anticipated that rogue exotic fish traders have released them in the lake to avoid being caught by anti-poaching forces.
Piranhas are normally about 15 to 25 cm long (6 to 10 inches), although some specimens have been reported to be up to 43 cm (18.0 inches) in length. Serrasalmus, Pristobrycon, Pygocentrus and Pygopristis are most easily recognized by their unique dentition. All piranhas have a single row of sharp teeth in both jaws; the teeth are tightly packed and interlocking (via small cusps) and used for rapid puncture and shearing. Individual teeth are typically broadly triangular, pointed and blade-like (flat in profile). There is minor variation in the number of cusps; in most species, the teeth are tricuspid with a larger middle cusp which makes the individual teeth appear markedly triangular. The exception is Pygopristis, which has pentacuspid teeth and a middle cusp usually only slightly larger than the other cusps. In the scale-eating Catoprion, the shape of their teeth is markedly different and the premaxillary teeth are in two rows, as in most other serrasalmines.