10 Surprising Facts About Piranhas

Piranhas are a crowd favorite in aquariums.
Piranhas are a crowd favorite in aquariums. / flyingv43/iStock via Getty Images

After sharks, piranhas are one of the world’s most misunderstood fish. They're found in freshwater rivers, ponds, and streams throughout northern South America, and they have a reputation for ferocity—even malice. In Hollywood, they’re often depicted as ravenous pack-hunters who won’t hesitate to eat their way through Bond villains and beach-going teenagers. But in reality, the fish usually try to avoid humans out in the wild. Here are 10 piranha facts that may surprise you.

1. Piranha attacks on humans are seldom fatal.

Bites usually happen out of the water: Captured piranhas may chomp on fishermen who try to extricate them from hooks and nets. For the most part, free-swimming piranhas in their natural habitat avoid people, though they may target human swimmers when water levels are low and food is scarce. This probably explains a 2017 incident in which 70 river-bathers were attacked by piranhas in Rosario, Argentina. (Everyone survived.)

Other attacks happen in places where food scraps have been dumped into the water. Male piranhas have also been known to lash out while guarding their eggs. Usually, the human victim is bitten only once and lives, though scientists think the fish aren’t above scavenging human corpses.

2. Schools of piranhas aren't trying to gang up on their victims.

Research suggests traveling in schools is a defensive strategy against predators like cormorants, caimans, and river dolphins. In a 2005 study, cormorant models were shown to groups of captive red-bellied piranhas. The fish who swam alone or in small clusters had faster breathing rates—a clear symptom of stress. By gathering in larger numbers, individual piranhas lower their odds of getting eaten.

3. Most piranhas are omnivores.

Piranhas don't really deserve their bloodthirsty reputation.
Piranhas don't really deserve their bloodthirsty reputation. / Sean Davis/iStock via Getty Images

Even well-known meat-eating species like the red-bellied and black piranhas can’t resist the occasional fruit, leaf, or fig. Other species eat mainly nuts and seeds. These fish remove the tough outer shells with their teeth before devouring the contents.

4. A few piranhas specialize in eating scales.

The scales and fins of many fish are 34 to 85 percent protein, but what’s more important is the fact that they’ll often grow back. In other words, they're a renewable food source: A piranha can repeatedly pluck scales off the same victim for years without killing it. The wimple piranha has turned the scale-grazing strategy into an art form. After it sneaks up on a target, it opens its jaws at a 120-degree angle and rips scales off the other animal’s sides. Though the wimple spends its entire life eating scales, some other piranhas will only feed on them as juveniles.

5. Proportionately, the black piranha has the strongest bite of any bony fish.

Native to the upper half of South America, the black piranha measures 8 to 15 inches long. A 2012 study found it can slam its muscular jaws with a bite force of 320 Newtons, three times stronger than an American alligator’s. Relative to its body size, the black piranha’s bite is more powerful than that of any other bony fish.

6. Some fish have piranha-proof armor.

South American arapaima can grow to be 10 feet long and weigh over 400 pounds. The slow-moving fish have little to fear from the piranhas who share their habitats. Arapaima scales are ultra-hard on the outside, but there’s a layer of flexible, cross-hatched fibers on the inside. When the scales get bent, they don’t crack, and lab experiments suggest that the inner fibers safely diffuse the force of a piranha bite.

7. Piranha teeth are replaceable.

Watch those fingers
Watch those fingers / milehightraveler/iStock via Getty Images

Piranha teeth measure up to 4 millimeters long. The upper and lower teeth interlock, allowing the piranha to slice through vegetation, meat, bone, and even metal. For that reason, piranhas shed the teeth on one side of their head—both lower and upper jaws—at once. A 2019 study in Evolution and Development found that piranhas grow a new set of teeth in their jaws and then swap out the old teeth for the new set several times during their lives.

8. Theodore Roosevelt contributed to piranhas' scary reputation.

During his travels in the Amazon, TR heard tales of the piranha's vicious nature and recounted them in his book Through the Brazilian Wilderness. “They are the most ferocious fish in the world,” Roosevelt wrote. "They will snap a finger off a hand incautiously trailed in the water; they mutilate swimmers ... they will rend and devour alive any wounded man or beast; for blood in the water excites them to madness." His descriptions, which echoed earlier accounts, convinced the American public that piranhas are wildly aggressive, man-eating river monsters. But as scientists now know, this characterization is false.

9. Steven Spielberg called Piranha (1978) “The Best of the Jaws Ripoffs.”

From Orca: The Killer Whale to Tentacles and Barracuda, a veritable school of aquatic horror movies tried to capitalize on Jaws’s success in the late 1970s. Piranha, a cult classic directed by Joe Dante, rode the wave of this trend. Some biologists have accused the flick—and its gory 2010 remake, Piranha 3-D—of contributing to the public’s negative perception of real piranhas.

10. Red-bellied piranhas have shown up in a few U.S. states.

Despite their bloodthirsty reputation, piranhas are a popular aquarium species. Unfortunately, some hobbyists dump them into waterways when they tire of them. They've been reported in 14 states, including Louisiana in 2021. There’s no evidence that red-bellied piranhas are establishing themselves as an invasive species. But if you're wading in a warm river or pond, you'd better watch your ankles.