Red-bellied piranhas mostly ambush fish from within the aquatic vegetation and dash after passing prey. A genome paper describes the species as mostly a carnivorous piscivore, a fish eater, that can be opportunistic and feed on plants, insects, worms and crustaceans. In the wild, a good portion of the diet is fins nipped from the tails of larger fish, and the fish also eats snails and other mollusks, and carrion. It is a scavenger in the Amazon basin. You may have heard that piranhas only eat meat. In fact, they also eat plants and fruit, and some piranha relatives are completely vegetarian.

Adult piranhas vary in color pattern and body size with geographic location, and at birth they are gray with pronounced black spots. A red-bellied piranha is a fish that lives in rivers in South America. It has a red belly, and its body is flecked with silver. FishBase gives a maximum length of 50.0 cm standard length for male or unsexed fish, a maximum published weight of 3.9 kg and a highly evolved capacity for hearing. Piranhas make sounds using muscles attached to the swim bladder. They are often found in whitewater streams in South America and often not in blackwater streams.

These fish swim in large shoals as 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, and the fish also tend to travel in schools of 20 or more. Inside the group, the younger piranhas are often toward the outside. 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.

Male piranhas build nests in the mud and sand at the bottom of the water. A nest is around 4 to 5 cm deep and 15 cm across. Spawning seems to occur during the wet season, and a courtship starts with swimming around in circles. Eggs and spawn are deposited in several batches and guarded by the male, sometimes assisted by the female. The eggs are laid in clusters and are stuck to plants growing under the water, and a female can lay thousands of eggs. In the wild, piranhas live in the Amazon River basin. The National Zoo says the red-bellied piranha is relatively common throughout its range, as of November 2026. They like warm water, about 25 degrees Celsius, and they can live about 10 years.

Words to know

• ambush: wait and surprise
• vegetation: water plants
• piscivore: fish eater
• spawning: laying eggs
• courtship: a pairing dance