Red-bellied piranha

A red-bellied piranha. Image taken at the California Academy of Sciences.

Of all of the fish in the Amazon, the piranhas are probably the ones with the most recognized name. Everybody has heard of piranhas. But what have they heard? What were your first impressions of piranha? Do you think they were accurate? Were they something like this:


Yes, that is a real movie (having been New World Picture’s most profitable movie, there is actually a part 2, Piranha 2: The Spawning. Not joking) .

So, this is the part where I tell you that piranhas are just misunderstood and they really aren’t dangerous and they are really beneficial and we should all love our piranha, right?

Well…not quite.

But I will talk about some of the things that science has discovered about them and some of the things that are myth, or at least lack authenticated records.

There are over 20 species of piranha each with a unique way of making a living. They are generally small animals, but there are at least 3 that are comparatively larger and could potentially kill or hurt a large animal or person. These are the black piranhas, the red-bellied piranhas and the pirayas. The largest of these are the pirayas. They can grow to be at least 20 inches long or more as adults. The black piranha’s bite is strong enough to remove a person’s finger or toe (more on this later). The red-bellied piranhas live mostly in lakes and are one of the most colorful piranhas. They have a bright red stomach and throat that becomes lighter as the fish gets older. In Brazil however, their bellies are yellow.

Red-bellied piranha

See the shiny red scales, belly and lower fins of a red-bellied piranha. Image taken at the California Academy of Sciences.

So, those are the 3 that are considered the most dangerous. However, to give this a little perspective, there are no verified, recorded human deaths from piranha attacks. Zero. So, why the reputation as people eaters? If you still doubt this reputation, from 1978 I give you:


So awesome. Anyway, there is probably a lot of reasons for the people eating fears. One of them is definitely the razor sharp jagged teeth that they have. They have triangular shaped, serrated teeth from birth. The lower part of their jaw juts out farther than the upper part of their jaw, so when they open their mouth, the teeth on their lower jaw point out. Their upper and lower teeth fit together like a puzzle, interlocking so no area is left unscathed from a piranha bite. And they are really sharp. They can bite through thin steel fishing hooks and fishing nets, and if they are flopping around on a boat with fishers who aren’t being careful, some can bite through a toe or a finger. (Despite this, they are heavily fished and eaten in the Amazon.) People in the Amazon sometimes use piranha jaws as cutting tools.

To make sure these weapons stay in tact, piranha species are known to shed and replace their teeth after a relatively short time compared to the functional life of the teeth. And to make sure that they can always eat, they replace their teeth on one side of their jaw at a time.

They use these sharp teeth to take small bites out of their prey’s bodies. They don’t chew, but swallow their food whole. Tiny but strong jaws mean that they literally nibble their prey to death. They will often take bites out of living animals, eating fins, tails and toes off of them while they swim. They will even bite off pieces of other piranhas’ fins. But, another interesting thing about them is that they’ll grow their fins back! Cichlids have actually developed a behavior that is specifically anti-piranha because of this. They will form a circle, with their tails pointing inward, so the piranha can’t easily bite their tails off.

There’s a few other reasons they might be cast as people eaters. What? Still not convinced they are unfairly cast in this role? Well then, from 2010 I give you:


Keep in mind, this is classified as sci-fi, horror and action, not humor. And that is at least 4 Piranhas-are-coming-to-kill you movies that I have found, and they are actually getting worse as they go.

Alright, so reason number 2 that they are considered people eaters is that…they…sort of…are. But not living people. Piranhas actually have a wide variety of feeding habits. In general, they are really not that picky. They’ll eat meat, fruits, nuts, seeds and sometimes plants that grow in the water. Young will eat insects. And while they will hunt (predominantly fish), they are also very likely to try to scavenge. A few researchers believe that people who have died in the water are scavenged upon by the fish, and when they are found, people attribute the cause of death to the piranhas. And the corpses that have been found can definitely be cause for fear or at least extreme respect. When researchers went to investigate, they found 3 cases to follow and they were all pretty gruesome. All of them had post-mortems where the people were found to have other causes of death (2 drowning, 1 heart attack) but were scavenged upon by the piranha. This is thought to fuel the piranha-as-man-eater image.

They are really efficient scavengers. One of the reasons why is that they have 4 nostrils and an amazing sense of smell. They can actually smell the blood of an injured animal (or human corpse) and are attracted to it. However, they’ll also scavenge for left over plant remains, such as leaves and seeds that other animals have left behind. This is an important job for the rain forest, as they will keep the rivers and lakes clean by eating dead animals and they will spread the seeds that they eat to new areas.

If reasons 1 and 2 weren’t terrifying enough, these fish scare humans one other way-they are often found traveling in schools, sometimes with over 100 fish swimming side by side. (They will not get too close to one another and they don’t like fish swimming directly behind them, probably because of the aforementioned fin and tail biting. If a fish gets too close, the offended fish might chase it or wag its body from side to side to tell it to get lost.) One piranha with pointy sharp teeth is scary enough, but over 100 piranhas with pointy sharp teeth is bone chilling.

So, a little bit about piranhas’ schooling behavior. First of all, not all piranhas are always in a school. Sometimes solitary piranhas do fine on their own and in fact make a handsome living as ambush predators as opposed to “pack hunters.” They will sometimes hunt together in their schools, using techniques such as having one piranha scatter a group of fish and then each piranha in the school can grab one for themselves. However, it is likely that they also school for protection. These animals are eaten by birds, people, snakes and crocodiles, so they have their own lives to worry about as well. (In fact, they also have a special type of scales called scutes along their belly, which when looked at closely form a saw-edge, which acts as extra armor against all of their potential predators.)

Another thing about their schooling behavior is that their school size is inversely proportional to the amount of water available. During droughts, when water levels drop, these fish “school” or gather in much larger groups wherever they can to survive. It is during this time that they have the hardest time getting food as well, and in this time when they are considered the most dangerous.