Red-bellied piranhas swim in large shoals. A shoal is a defence against their many predators, including dolphins, larger fish and birds. A 2005 study concluded that shoaling has a cover-seeking function in this species. Some groups have more than a hundred fish in them. They tend to travel in schools of 20 or more. Inside the group, the younger piranhas are usually toward the outside of the group, and the larger fish that are sexually mature are in the center. You may have heard that piranhas hunt in packs. In fact, they gather in groups for protection rather than hunting their prey in packs.

A red-bellied piranha has a red belly, and its body is flecked with silver. At birth, piranhas are gray with pronounced black spots. A piranha grows to about a foot long. It can weigh up to 4 pounds, or 1.81 kilograms. It lives in the Amazon River basin, the Paraguay-Paraná River basin and rivers of northeastern Brazil. It is most often found in whitewater streams in South America, and not often in blackwater streams. It likes warm water, about 25 degrees Celsius. A piranha can live about 10 years.

A genome paper calls the red-bellied piranha mostly a carnivorous piscivore, which means a fish eater. It says the fish can be opportunistic and feed on plants, insects, worms and crustaceans. It also eats snails and other mollusks and carrion. It is a scavenger in the Amazon basin. In the wild, a good portion of its diet is fins nipped from the tails of larger fish. You may have heard that piranhas only eat meat. In fact, they also eat plants and fruit. Some piranha relatives are completely vegetarian.

In the wild, male piranhas build nests in the mud and sand at the bottom of the water. Spawning, the laying of eggs, seems to occur during the wet season, and a courtship starts with swimming around in circles. A female can lay thousands of eggs. The eggs are laid in clusters and stuck to plants growing under the water. FishBase lists a highly evolved capacity for hearing. Red-bellied piranhas also make sounds using muscles attached to the swim bladder. The National Zoo says the red-bellied piranha is relatively common throughout its range, as of November 2026.

Words to know

• shoal: fish group
• defence: staying safe
• spawning: laying eggs
• courtship: a pairing dance
• piscivore: fish eater