Taxonomy
Piranhas belong to the subfamily Serrasalminae, which also includes closely related herbivorous fish including 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 Catoprion than the other three piranha genera.
The total number of piranha species is unknown and new species continue to be described.

Piranhas are found only in the Amazon basin, in the Orinoco, in rivers of the Guyanas, in the Paraguay-Paraná, and in 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.

However, piranha (inevitably former aquarium-dwellers) have been introduced into parts of the United States, even being occasionally found in the Potomac River, although they typically do not survive the cold winters of that region. Piranha have also been discovered in the Kaptai Lake in south-east Bangladesh. 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.

Description
Piranhas are normally about 15 to 25 cm long (6 to 10 inches), although reportedly individuals have been found 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.

Ecology
Ecologically, piranhas are important components of their native environments. Although largely restricted to lowland drainages, these fish are widespread and inhabit diverse habitats within both lotic and lentic environments. Some piranha species are abundant locally, and multiple species often occur together. As both predators and scavengers, piranhas influence the local distribution and composition of fish assemblages. Certain piranha species consume large quantities of seeds, but unlike the related Colossoma and Piaractus, herbivorous piranhas thoroughly masticate and entirely devour all seeds eaten and consequently do not function as dispersers.

The piranha is all-too-frequently portrayed as a vicious species of fish hunting in large schools. This conception was created from the belief that piranhas created schools for hunting purposes. Recent research, however, suggests that this is actually used as a defense mechanism against the piranha's natural predators (such as river dolphins, caimans and gigantic arapaima).

Research on the species Serrasalmus aff. brandtii and Pygocentrus nattereri in Viana Lake, which is formed during the wet season when the Rio Pindare (a tributary of the Rio Mearim) floods, has shown that these species eat vegetable matter at some stages in their life; they are not strictly carnivorous fish.

Relationship to people
Locals often use piranha teeth to make tools and weapons. Piranha are also a popular food, although if an individual piranha is caught on a hook or line, it may be attacked by other (free) piranhas.

Piranha are commonly consumed by subsistence fishermen and often sold for food in local markets. In recent decades, dried specimens have been marketed as tourist souvenirs. Piranhas occasionally bite and sometimes injure bathers and swimmers, but truly serious attacks are rare and the threat to humans has been largely exaggerated. For example, in the James Bond film You Only Live Twice, the villain has a pool of piranha fish which can strip a human body to the bone.  A piranha bite is sometimes considered more an act of carelessness than that of misfortune, but piranhas are a considerable nuisance to commercial and sport fishers because they steal bait, mutilate catch, damage nets and other gear and may bite when handled.

Several piranha species appear in the aquarium trade. Piranhas can be bought as pets in some areas, but they are illegal in much of the United States including Alabama, Alaska, Arizona, Arkansas, California, Colorado, Florida, Georgia, Hawaii, Kentucky, Louisiana, Maine, Massachusetts, Mississippi, Nevada, New Mexico, New York, North Carolina, Oklahoma, South Carolina, Texas, Utah, Virginia and Washington.
The most common aquarium piranha is the Pygocentrus nattereri, or the red-bellied piranha. Piranhas can be bought fully grown or as babies, often no larger than a thumbnail. It is important to keep Pygocentrus piranhas alone or in groups of four or more, not in pairs, since aggression among them is common and is distributed more widely when kept in larger groups, not allowing the weaker fish to survive. It is not rare to find individuals with one eye missing due to a previous attack. If underfed, piranhas are likely to become cannibalistic on others in their group.

Source: wikipedia.org, Copyright (c) 2009.

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