They are normally about 15 to 25 cm long (6 to 10 inches), although
reportedly individuals have been found up to 40 cm in length. They
are known for their sharp teeth (able to bite through a steel fishing
hook) and an aggressive appetite for meat and flesh. They are normally
only found in the Amazonian, Guianas and Paraguayan river systems.
However, piranha (most likely former aquarium-dwellers) are also
occasionally found in the Potomac River, but they typically do not
survive the cold winters of that region
Recent research on Serrasalmus aff. brandtii and Pygocentrus nattereri
in Viana Lake, which is formed during the wet season when the Rio
Pindare (a left bank tributary of the Rio Mearim) floods, has shown
that these species eat vegetable matter at some stages in their
life history. They are not strictly carnivorous fishes.
Piranhas generally pose little threat to humans, and attacks on
humans are extremely rare. Natives frequently swim in piranha infested
water without attacks or scratches. However, it is not recommended
to swim where piranha live in drought season because of increased
aggressiveness caused by food scarcity and increased tendency to
form large schools. Piranha fish also have the same sensory system
that enables sharks to detect blood in minuscule amounts, so it
is believed that swimming with an open cut may enhance the chance
of an attack.
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