Piranha
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[Photo] Manuel's Piranha or Green Tiger Piranha (Serrasalmus manueli). Juvenile Green Tiger Piranha (Serrasalmus manueli), 6,5 inches in length. Private aquarium Groningen, The Netherlands, July 2005. Taken with Sony DSC-P73. Photographer: Jonas Hansel (jonashansel@gmail.com) www.piranha-info.com

The piranhas or pira??as (IPA pronunciation: /p??i??an??j??/ (or /p????an??j??/), /p??i??a??????/, or /p??i??an????/) are a group of omnivorous freshwater fish living in South American rivers. In Venezuelan rivers they are called caribes. They are known for their sharp teeth and an aggressive appetite for meat and flesh.

The name piranha may come from a hybrid language composed of Tupi-Guarani languages; it may be a compound word made of the components 'pir??', meaning 'fish', and 'sanha' or 'ranha', meaning 'tooth'. In Tupi, inalienably possessed nouns take the prefix 't-', 's-', or 'r-' depending on the possessor, or zero in combination; thus 'pir??'+'anha'. Alternatively, it may come from Tupi 'pir??' ('fish') and '??nha' (devil).

Taxonomy
Piranhas belong to the subfamily of Serrasalminae (which also includes closely related herbivorous fish including pacus and silver dollars). 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 number of piranha species is not known, and new species continue to be described. In 1988, it was stated that fewer than 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.

Distribution
Piranhas are found only in the Amazon basin, in the Orinoco, in rivers of the Guyanas, in the Paraguay-Paran??, and in the S??o Francisco River systems; some species of piranha have extremely broad geographic ranges, occurring in more than one of the major basins mentioned above, whereas others appear to have much 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, but they typically do not survive the cold winters of that region. Recently a piranha was caught by a fisherman in the Catawba River in North Carolina. This is the first known case in North Carolina and possibly in the region.

Description
They are normally about 15 to 25 cm long (6 to 10 inches), although reportedly individuals have been found up to 41 cm (24 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 that makes the individual teeth appear markedly triangular. The exception is Pygopristis, which has pentacuspid teeth and a middle cusp that is 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 fishes 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 destroy all seeds eaten and consequently do not function as dispersers.

The piranha is renownly portrayed and known as a vicious species of fish hunting in large schools. This conception was created from the past 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 giant pirarucu.

Recent on 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 humans
Locals use their teeth in tools and weapons. Piranha are also a popular food, though if caught on a hook or line it may be attacked by other piranhas.

Piranha are commonly consumed by subsistence fishers and frequently 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. However, 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.

A few piranha species occasionally appear in the aquarium trade. Piranhas can be purchased as pets in some areas; however, they are illegal in some parts of the United States, such as the State of Washington. The most common piranha is the Pygocentrus nattereri, or the red-bellied piranha. Piranhas can be bought fully grown or as babies, often no bigger than a thumbnail. It is important to keep Pygocentrus piranhas either singularly or in groups of three or more, rather than simply pairs, since aggression amongst the group is common and distributed more widely when kept in larger groups, allowing the weaker fish to survive. When kept in groups, it is recommended that they are in even-numbered groups, as piranhas will gang up on an odd member. While any fish-based foods are adequate for feeding, thawed shrimp, fillets of white fish, and disease free feeders are preferred. The young are to be fed very little, as overfeeding can kill them. However, they will eat more as they grow older and larger. In order to provide a balanced diet, it is usually necessary to change types of food often. Feeder goldfish are a popular choice for feeding piranhas, although they contain a B vitamin inhibitor that may stunt growth and shorten the fish's life span. Piranhas prefer a darker environment with a lot of plant cover, as they become agitated when denied appropriate cover.

http://en.wikipedia.org/wiki/Piranha

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