12 Piranha Facts to Sink Your Teeth Into

Red-bellied piranha (Pygocentrus nattereri). Sylvain Cordier / Getty Images

Piranhas’ reputation precedes them. These feisty South American fish are notorious for their sharp teeth, fierce demeanor, and outsized appetites, which allegedly can compel a group of piranhas to skeletonize a cow in minutes.

Yet while they are a potent force in their native waterways, piranhas are also far more diverse—and less dangerous to people and cattle—than commonly believed.

In hopes of shedding more light on these misunderstood fish, here are a few interesting and lesser-known facts about piranhas.

1. Piranhas Pose Little Risk to People

Piranha attacks on humans are rare, and when they do occur, typically involve one or just a few bites to the hands or feet by a single fish, resulting in injuries that are painful but not life-threatening. There are very few documented cases of piranhas consuming a human, and at least three of those involved people who had already died from drowning or other causes.

The risk of piranha bites may increase at times when food is scarce, or if swimmers get too close to their spawn in the riverbed. According to a study of piranha attacks in Suriname, bites were associated with high densities of piranhas during the dry season, high densities of people, commotion in the water caused by people, and the spilling of food or blood into the water.

2. They Are Surprisingly Diverse

Redeye Piranha (Serrasalmus rhombeus)
Redeye Piranha (Serrasalmus rhombeus). Antagain / Getty Images

Piranhas belong to the taxonomic family Serrasalmidae, along with related fish known as pacu and silver dollars. There is no clear consensus about the number of piranha species alive today, due to challenges in identifying species, linking juveniles with adults, and unraveling their evolutionary histories, as researchers wrote in a study published in the journal Zootaxa.

That said, we know piranhas are a diverse group of fish with a wide array of diets and behaviors. Estimates range from as few as 30 to as many as 60 species of piranhas, all native to rivers and lakes in South America.

3. We Don’t Really Know When They Evolved

Modern piranhas might have evolved as recently as 1.8 million years ago, around the beginning of the Pleistocene Epoch, according to the Zootaxa study. Other research suggests the main piranha lineages diverged from their most recent common ancestor about 9 million years ago, during the Miocene Epoch. That was around the same time South America was home to the now-extinct “megapiranha” (see No. 9 below).

4. Many Piranhas Eat Plants

Red-bellied Piranha, Pygocentrus nattereri, at the Georgia Aquarium
Red-bellied Piranha (Pygocentrus nattereri). Ed Reschke / Getty Images

Despite their stereotype as bloodthirsty carnivores, piranhas are classified as omnivores, since most species eat at least some plant material—and some may even be vegetarian. The red-bellied piranha (Pygocentrus nattereri), for example, is widely known as a ferocious predator, but it’s really an omnivorous forager and scavenger, feeding on fish, insects, crustaceans, snails, and plants. In fact, a study of red-bellied piranha stomach contents found plants to be their second food item, behind only fish.

Piranha diets tend to be flexible, often changing throughout a fish’s life as it grows and as resources wax and wane. Seeds, leaves, and other plant material might sustain a piranha as it hunts for heartier food, and could be seasonally vital. Tometes camunani, a species discovered in 2013, has been described as a phytophagous (plant-eating) piranha that feeds mainly on riverweeds in the family Podostemaceae.

5. Some Specialize in Eating Scales

Fish are a big food source for many piranhas, but falling victim to a piranha isn’t always fatal for their prey. Opportunistic piranhas will make do with a fin or some scales from the ones that got away, and some species are specialist scale eaters, having adapted to feed primarily on the scales of other fish.

Scale eating, also known as lepidophagy, has evolved independently in a few fish lineages. It’s reportedly more common among young piranhas, although some species remain focused on scales in adulthood, often using specialized hunting techniques. The wimple piranha (Catoprion mento), for one, uses a “high-speed, open-mouth, ramming attack,” as researchers wrote in the Journal of Experimental Biology, biting upon impact to remove scales with its teeth while also knocking them loose with the force of its collision.

6. Piranhas Swarm for Safety, Not Hunting

piranhas in an aquarium, Germany
OlgaMironova / Getty Images

Although piranhas are famous for their feeding frenzies, in which a large group quickly rips a much larger animal to shreds, that doesn’t seem to be normal behavior. Their live prey is typically smaller, and they aren’t known to hunt in large groups.

The red-bellied piranha is one species often credited with overwhelming large prey, but while the species does sometimes travel in groups called shoals, research suggests this behavior is less about finding prey than avoiding their own predators. Based on experiments with wild-caught piranhas and simulated predators, the authors of one study published in Biology Letters concluded that “shoaling has a cover-seeking function in this species.”

7. They Make Sounds to Communicate

Red piranha, Pygocentrus nattereri
Red piranha (Pygocentrus nattereri). Henrik Sorensen / Getty Images

Some piranhas are noisy when handled; red-bellied piranhas, for example, famously “bark” (and sometimes bite) in the hands of anglers who catch them. Not much was known about these sounds until recently, when researchers discovered the species can make three distinct sounds, each for a different situation.

The aforementioned barks were associated with frontal displays, in which piranhas stare each other down for intimidation. Once two piranhas begin actively circling or fighting, barks may give way to a low grunting or thudding sound, which the researchers suspect is more threatening. Both of those sounds are made with the piranha’s swim bladder, while a third gnashing sound is made with the teeth during chasing behavior.

8. They Have an Outsized Bite Force

Serrasalmus rhombeus (Redeye Piranha, Peruvian Black Piranha)
Redeye Piranha, Peruvian Black Piranha (Serrasalmus rhombeus). ilbusca / Getty Images

Piranhas may not be the vicious monsters depicted in movies, but they do have a vicious bite for their size. One of the largest modern species, the black or redeye piranha (Serrasalmus rhombeus), has a bite force of 320 Newtons, according to a 2012 study published in Scientific Reports. That is “the strongest yet recorded for any bony or cartilaginous fish to date,” the study’s authors wrote, noting it’s nearly triple the bite force of an equivalent-size American alligator.

9. The Extinct ‘Megapiranha’ Had Zigzag Teeth

Modern piranhas have a single row of sharp teeth, while their closest living relatives, the pacus, have two rows of flatter teeth. Scientists had suspected their last common ancestor would have two rows of teeth, which eventually merged in piranhas, and in 2009, a study published in the Journal of Vertebrate Paleontology revealed a previously unknown species (and genus) that fit the bill.

Named Megapiranha paranensis, the now-extinct fish is known only from a piece of fossilized jawbone. That fossil included a row of zigzag teeth, the expected arrangement for a transitional species moving from two rows of teeth to one. Megapiranha was slightly larger than the largest modern piranhas, with an estimated length of about 3 feet, and also boasted powerful jaws. Based on fossil reconstructions and simulations, researchers have described Megapiranha as “a ferocious bone-crushing mega-predator of the Miocene epoch.”

10. Piranha Means ‘Biting Fish’

Piranha Serrasalmus Teeth
Piranha Serrasalmus. Staffan Widstrand / Getty Images

The original name for piranhas was pira nya, or “biting fish,” among the indigenous Tupi people in what is now Brazil, according to the Online Etymology Dictionary. Portuguese settlers adopted the term from the Tupi language, but with the modified spelling piranha.

In Portuguese, “nh” is pronounced like “ñ” in Spanish, so piranha preserves the “nya” sound of the Tupi word. So does piraña in Spanish, which produces the same sound with a tilde. English retains the spelling of the Portuguese word, although English speakers now commonly pronounce it more like “pirahna.”

11. Teddy Roosevelt Played a Role in Vilifying Them

In his 1914 book “Through the Brazilian Wilderness,” former U.S. President Theodore Roosevelt recounted his recent adventures and calamities exploring the River of Doubt in the Amazon rainforest. One animal that seemed to make an especially strong impression on Roosevelt was the piranha, which he described as a “blood-crazy fish” and “the embodiment of evil ferocity.”

However, this may have been at least partly based on a misleading experience Roosevelt had with piranhas, according to a report by the late tropical fish expert Herbert R. Axelrod. To create a spectacle for the visiting dignitary, local people reportedly spent weeks catching piranhas and holding them in a netted-off section of river without food, then pushed an old cow into the river for Roosevelt to see them devour it.

12. Piranhas Are Important

Jabiru Stork
Jabiru Stork with freshly caught piranha in the Northern Pantanal, State Mato Grosso, Brazil. thejack / Getty Images

Piranhas are not the apex predators we imagine them to be, but they still play valuable roles in their native ecosystems as mesopredators, scavengers, and prey. They are widespread and sometimes locally abundant across a large swath of South America, giving them broad ecological influence.

By so actively hunting and scavenging in their habitats, piranhas help shape the local distribution and composition of fish as well as other wildlife. And since they’re relatively small, and not quite the unstoppable evil described by Roosevelt, they also provide an important food source for other predators, like herons and cormorants.

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