Fish

Piranha

Serrasalmidae · also called piranhas, caribe

Not Evaluated carnivoreomnivore Rivers & wetlands

The piranha’s reputation is almost entirely a story a US president told. In 1914 Theodore Roosevelt watched a cow thrown to a pen of deliberately starved piranhas and wrote it up as a normal river hazard, and the image of water that boils with flesh-eating fish has stuck ever since. The real animal is a wary, mostly fruit-and-fin-eating fish that shoals for its own protection.

Overview

Piranhas are the meat-eating members of the family Serrasalmidae, in the fresh waters of tropical South America. Of around 60 species, only a few — chiefly the red-bellied and black piranha — are notably predatory. Their plant-eating relatives, the pacus, are in the same family. Diet is varied — fish, fins, insects, carrion, and a large amount of fruit and seeds.

The teeth and the bite

A piranha has a single row of interlocking triangular teeth that mesh like scissors, replaced in quarter-jaw blocks so it is never toothless. The black piranha bites with about 320 newtons — around 30 times its body weight, one of the strongest bites for its size of any animal.

Shoaling for defence

Piranhas gather in groups of dozens to hundreds, and the main purpose is protection from caimans, dolphins, and big fish — a piranha in the middle of a shoal is calmer and safer than one at the edge. They also bark and croak using a muscle against the swim bladder.

Danger, in proportion

Bites on people are real but are single defensive nips to a hand or foot in shallow, warm dry-season water where fish are crowded and guarding nests — not swarming attacks. Fatal cases almost always involve someone already dead in the water.

Status

Piranhas as a family are Not Evaluated. Most are common and widespread, several are food fish, and none is listed as threatened, though some pacus are locally overfished and dams and deforestation alter the flooded-forest habitat many depend on.

Piranha: common questions

Do piranhas really strip animals to the bone in minutes?

Almost never in the wild. A very large shoal of hungry piranhas concentrated in a shrinking dry-season pool can quickly consume a dead or dying animal that falls in, and this does happen, but it is the exception, not the rule. Piranhas are not normally in a frenzy, they do not hunt large mammals, and a healthy animal swimming across a river is at little risk. The famous stripped-cow image traces to a single staged event laid on for Theodore Roosevelt in 1914, using piranhas that had been deliberately starved and penned. Under ordinary conditions piranhas are wary fish that scatter when something large enters the water.

How dangerous are piranhas to swimmers?

The real risk is low but not zero. Bites on people do occur, most often in shallow, warm, still or slow water during the dry season, when piranhas are crowded together and some species are guarding nests and will nip a foot or hand that comes close. These are single, defensive bites that remove a chunk of flesh and need stitches, not a swarming attack. Fatal attacks are extremely rare and usually involve a person who was already dead or unconscious in the water. Millions of people live and swim alongside piranhas every day. Avoiding shallow murky water in the dry season and staying away from spawning areas removes most of the risk.

What is the difference between a piranha and a pacu?

They are close relatives in the same family, and the main difference is the teeth and the diet that goes with them. A piranha has sharp, pointed, triangular teeth in a single interlocking row and a strongly shearing bite, suited to cutting flesh, fins, and scales. A pacu has broad, flat, molar-like teeth, sometimes unnervingly similar to human teeth, and powerful jaws built for crushing the fruit, nuts, and seeds that fall into the flooded forest, which make up most of its diet. Pacus grow much larger than most piranhas, up to a metre and 25 kilograms, and are important food and aquaculture fish. A pacu can give a hard crushing bite but is not a flesh-eater.

Can piranhas live outside South America?

Only where the water stays warm year-round. Piranhas released from home aquariums have been caught in lakes and rivers in many countries, including the United States, Britain, and China, and there are periodic scares when one turns up. But piranhas cannot tolerate cold water, and in temperate regions the winter kills them before they can establish a breeding population, so these are almost always isolated escaped pets rather than a spreading invasion. In genuinely tropical areas outside their native range, such as parts of Bangladesh and China, small established populations have been reported, and keeping or importing piranhas is banned or restricted in many places for this reason.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 10, 2026. Last updated September 10, 2026.

Quick facts

Scientific name: Serrasalmidae
Diet: Far more varied than the reputation suggests, with piranhas eating fish, fins nipped from larger fish, insects, crustaceans, snails, worms, and carrion, and a large share of many species' diet being fruit, seeds, and plant matter, while their close relatives the pacus are mostly fruit and nut eaters
Teeth that work as a single blade: A piranha has a single row of flat, triangular, razor-edged teeth in each jaw that interlock when the mouth closes so the upper and lower sets mesh like scissors, and the teeth are replaced in blocks of a quarter jaw at a time, so a piranha is never toothless
One of the hardest bites for its size: The black piranha can bite with a force of around 320 newtons, roughly 30 times its own body weight and, pound for pound, one of the most powerful bites measured in any living animal, an adaptation for shearing through the fins, scales, and bones of prey and for cracking hard fruit
They shoal for safety, not to hunt: Piranhas gather in groups of dozens to hundreds, and research shows the main function is defence against their own predators, which include caimans, river dolphins, large fish, and birds, with piranhas in the centre of a shoal calmer and safer than those at the edge
A myth started by a president: The image of a river that can strip a cow to bone in minutes comes largely from Theodore Roosevelt's 1914 account of a staged spectacle in Brazil, where locals had penned starving piranhas for days and thrown in a sick cow for the visiting dignitary, and Roosevelt reported it as normal piranha behaviour
They bark and croak: Piranhas produce several distinct sounds by rapidly vibrating a muscle against the swim bladder, including a bark when confronting another fish, a low drumming during a chase, and a croak made with the jaws, which are used in disputes and to space out a shoal
Only a handful of species are aggressive: Of around 60 piranha species only a few, chiefly the red-bellied piranha and the black piranha, are notably predatory and capable of biting a person, and bites on humans, while real, are almost always single defensive nips to a hand or foot in shallow, low, warm water during the dry season, when fish are crowded and guarding nests
Range: The fresh waters of tropical South America east of the Andes, in the Amazon, Orinoco, Paraguay, Parana, and Sao Francisco river systems and the rivers of the Guianas, in main river channels, oxbow lakes, floodplain lakes, and seasonally flooded forest, with piranhas also introduced, mostly from aquarium releases, to warm waters in the United States, Europe, and Asia, where cold winters usually prevent them establishing.
Conservation status: Not Evaluated ≈ Not assessed as a family of around 90 species including the pacus, and most piranhas are common and widespread across the rivers of tropical South America, with several important as food fish and none currently listed as globally threatened, though some pacus and larger serrasalmids are locally overfished, and dams and deforestation alter the flooded-forest habitat that many depend on for feeding and spawning in the wild