I’ve written about some pretty fearsome fish on this blog; barracudas, goliath tigerfish, and electric eels are some that come to mind. Yet it seems I’ve neglected a fish with a reputation to put all the rest to shame. I think it’s about time I wrote about piranhas.

There are between 30 and 60 species of piranhas, but I am going to focus on the red-bellied piranha, as they are the species that have the most fearsome reputation. Red-bellied piranhas are found in South America, in whitewater streams east of the Andes. They occur in every country in South America except for Suriname and Chile.

Red-bellied piranhas grow to be half a meter in length, and can weigh up to 4 kilograms. Adult piranhas have characteristic red colouring on their bellies, while young fish are simply silver with dark spots. They have strong jaws and nasty, sharp teeth.

The jawbone of a red-bellied piranha. Check out those teeth! Image by Sarefo, CC BY-SA 3.0, via Wikimedia Commons

While most people consider piranhas to be vicious flesh-tearing creatures, they are actually omnivorous scavengers. They mainly eat worms, insects, crustaceans and other fish. They will also consume debris, fish fins and scales, and plants. Young fish hunt during the day, while larger piranhas are active in the late afternoon and evening. Their primary method of feeding is ambush hunting, where they wait in vegetation and dark areas of the water to surprise a victim.

Though it is often thought that piranhas gather in large schools to attack big prey (a cow, for example), they actually do not hunt in groups. Red-bellied piranhas do form schools. But these are as a defence against predators, not for hunting. Stories of piranhas stripping a cow carcass in minutes are the exception — these types of events occur when the piranhas have been starved and food is very scarce. That being said, piranhas have attacked people, with some recorded fatalities. These occur when water levels are lower, and there is less food and piranha densities are higher.

Red-bellied piranhas use sounds to communicate with each other, using sonic muscles and their swim bladder to produce noises. They mainly use these sounds during aggressive interactions with one another. Low, drumming sounds are signals of mild or moderate attacks, while louder sounds are made during fierce attacks. So if you happen to hear a short, loud noise in a river in South America, get out of the water.

Look at the cute piranhas! Image by FF23-fr, CC BY-SA 3.0, via Wikimedia Commons

Mating in piranhas coincides with rises in water levels and floods in the rivers they inhabit. Females lay thousands of eggs in a nest four to five centimetres in depth, near the bottom of water plants. Females and males will swim closely together in circles; this may be a courtship display but is more likely the two parents defending the nesting site. Fish eggs are pretty tasty meals, but I’m not sure I’d want to go after some guarded by piranhas. The eggs hatch after two to three days, and the young will remain hidden in the plants until they are large and fierce and able to fight off any hungry fish.

Red-bellied piranhas are not currently endangered or threatened, though capture for aquaria may put some pressure on the species. Yes, that’s right, people like to keep piranhas as pets. Personally I wouldn’t want a piranha in my house, but I guess some people like them.

Cover image by Tiia Monto, CC BY-SA 4.0, via Wikimedia Commons