Piranha: Fish With a Vicious Reputation and Mixed Diet

Chances are, if you've seen a piranha at an aquarium, it's a red-bellied piranha. In fact, the total number of piranha species is a matter of some debate. But red-bellied piranhas are the most commonly found, especially in captivity, which is also the species most likely to be sensationalized in media and folklore.

The only natural home of piranhas is in South American rivers such as the Amazon, but they have been introduced into freshwater systems in other parts of the world, mostly through the (usually criminal) tropical fish trade. They are illegal to keep as pets in most U.S. states, although most piranha species wouldn't be able to survive in a place much colder than Florida anyway.

We don't know a lot about the habits of Piranhas in the wild; much of what we know, we learned from keeping them in aquaria, and most animals tend to act differently in captivity. The largest of modern piranha genera grow to be about 2 feet (0.6 meters) in length, but many species max out at about half that length.

Their jaws are lined with a single row of saw-like interlocking teeth that resemble shark teeth — razor-sharp and always exposed, due to a pronounced underbite.

Most piranha eat meat, but just because you have a jaw three times more powerful than a similarly sized alligator and can bite through the plates of the most heavily armored fish of the Amazon doesn't mean you have to eat meat all the time.

Piranhas, like most wild animals, eat what they can get. A typical piranha diet includes a lot of aquatic invertebrates (worms and insects), other fish, some carrion they find in the river, as well as seeds and vegetation.

Depending on the species, the ratio of these elements vary: Some types of piranha are entirely vegetarian, while others, like the infamous red-bellied piranha, reportedly wait in the weeds every evening in groups of 20 to 30 to ambush prey.

An underwater environment is full of sounds just like the ones in your neighborhood, and in the Amazon, Piranhas are pretty noisy neighbors. They make distinctive "barking" noises by squeezing their swim bladders into different shapes.

According to Rodney Rountree, a marine biologist and an expert in fish sound production, who has studied piranha vocalizations in the Amazon basin in Peru, the patterns of each species' underwater barks are very distinctive, which can help scientist looking for piranha in murky waters figure out which species are in an area without even catching them — all they needed is a mic in the water.

Not only do different piranha species vocalize differently, piranhas make different types of noises to communicate different messages. For instance, a "bark" might be a warning for another fish to back off, while a group of piranhas may emit an intimidating chorus of grunts as they gather around a prey item.

"You hear all sorts of fish in the soundscape," says Rountree. "There are many different species of catfish, for instance. In the areas where I was catching piranhas for my research, I was hearing other fish sounds, and it became obvious that it was the sound of other fish screaming because they were being bitten by piranhas."

Although you don't want to tangle with the mouth end of a piranha, a lot of popular lore around these fish can be a bit overblown. For instance, that classic situation in movies wherein a cow steps into a river and is immediately pulled under and stripped to the bone by a pack of hungry piranhas isn't something that happens in real life.

The waters of the Amazon are often very opaque — full of silt and tannins — and fish can't see very well, so they find their prey by smell or proximity. Most piranhas are "nippers," meaning they feed by sneaking up on their prey and shearing off a single chunk of flesh. It doesn't much matter to the piranha who it is; they'll even take bites out of each other.

According to Rountree, this "nipping behavior" isn't unique to piranhas. In fact, it's fairly common among fish in the Amazon.

"There are all kinds of fish that just take a little chunk out of other fish," he says. "Many of the fish are armored — especially some of the catfishes have heavy, bony scales — and part of this is for defense."

The modern myth of the piranha was turbo-charged with the release of "Piranha" in 1978. This B-horror flick helped to cement the idea of a fish with razor-sharp teeth that lurked in the torrid waters of South America, ready to devour alive anyone foolish enough to go for a dip.

The truth is quite a bit less sensational. While it's true that piranha teeth are sharp enough to cut through flesh, piranhas habitually attack things that are easy wounded or dead. While it would certainly be unpleasant to get even a nip from a piranha, it's unlikely any healthy person has even been devoured during a feeding frenzy.

Most piranha attacks are nonlethal. While piranhas are known to go after large animals, it's almost always because those animals — human or not — are either very close to dying or already dead, like wounded wild fowl or even capybaras.

The Amazon is full of fearsome underwater killers — electric eels, for one, are arguably a good deal more deadly than red-bellied piranhas. However, in 1913, former U.S. president Teddy Roosevelt took a pleasure cruise in the Brazilian Amazon, and it very nearly killed him.

At one settlement, locals showed Roosevelt a pool full of starving piranhas and reportedly fed a live cow to the fish to show off their ferocity. Roosevelt was really struck by their killing prowess and wrote about the formidable fish in his journal, describing the experience and calling them, "the most ferocious fish in the world."

According to Rountree, while a piranha might take a single chunk out of you, the devouring of a cow is virtually unheard of: