Although often described as extremely predatory and mainly feeding on fish, their dietary habits vary extensively, and they will also take plant material,[3] leading to their classification as omnivorous.

[4] The name originates from Old Tupi pirãîa,[5] being first attested in the 1587 treatise Notícia do Brasil [pt] by Portuguese explorer Gabriel Soares de Sousa.

[9] Traditionally, only the four genera Pristobrycon, Pygocentrus, Pygopristis, and Serrasalmus are considered to be true piranhas, due to their specialized teeth.

Some rogue exotic fish traders are thought to have released them in the lake to avoid being caught by antipoaching forces.

Relative to body mass, the black piranha (Serrasalmus rhombeus) produces one of the most forceful bites measured in vertebrates.

[3] Although popularly described as highly predatory and primarily feeding on fish, piranha diets vary extensively,[3] leading to their classification as omnivorous.

[4] In addition to fish (occasionally even their own species[20]), documented food items for piranhas include other vertebrates (mammals, birds, reptiles), invertebrates (insects, crustaceans), fruits, seeds, leaves and detritus.

[21] In another study of more than 250 Serrasalmus rhombeus at Ji-Paraná (Machado) River, 75% to 81% (depending on season) of the stomach content was fish, but about 10% was fruits or seeds.

[3] Piranhas will often scavenge,[9] and some species such as Serrasalmus elongatus are specialized scale-eaters, feeding primarily on scales and fins of other fish.

This practice has been documented among several South American tribes including the Camayura and Shavante in Brazil and the Pacahuara in Bolivia.

However, they are often considered a nuisance by fishers because they steal bait, eat catches, damage fishing gear, and may bite when accidentally caught.

[3][29] Swimming near fishermen may increase the risk of attacks due to the commotion caused by struggling fish and the presence of bait in the water.

[33] Most piranha attacks on humans only result in minor injuries, typically to the feet or hands, but they are occasionally more serious and very rarely can be fatal.

[29] Near the city of Palmas in Brazil, 190 piranha attacks, all involving single bites to the feet, were reported in the first half of 2007 in an artificial lake which appeared after the damming of the Tocantins River.

[31] In 2011, another series of attacks at José de Freitas in the Brazilian state of Piauí resulted in 100 people being treated for bites to their toes or heels.

[37] In January 2015, a six-year-old girl was found dead with signs of piranha bites on part of her body after her family canoe capsized during a vacation in Monte Alegre, Brazil.

[38] Whereas fatal attacks on humans are rare, piranhas will readily feed on bodies of people that already have died, such as drowning victims.