Scientists have sequenced and assembled the red-bellied piranha genome. In one genome study, red-bellied piranhas matured at about 16 cm total length. At birth, the fish are gray with pronounced black spots, and an adult has a red belly and a body flecked with silver. Adult piranhas vary in color pattern and body size with geographic location. The 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. The species is often found in whitewater streams in South America and often not in blackwater streams.

In a 2020-2021 study of piranhas in a Brazilian river, fish remains were the most relevant food items at all the collection points. A Western Kentucky University team argued that piranhas eat mostly other fish fins, not giant fish that could easily swallow them whole, and biologist Steve Huskey says piranhas get the vast majority of their nutrients by clipping the fins of other fish. In the wild, a good portion of the piranha's diet is fins nipped from the tails of larger fish. One study suggests the plant intake of the piranha may be an active way of gaining food supplies while scanning for prey. You may have heard that piranhas only eat meat. In fact, piranhas also eat plants and fruit. In the wild, the species mostly ambushes fish from within the aquatic vegetation and dashes after passing prey.

Piranhas make sounds using muscles attached to the swim bladder, and the pitch of their bark and drum sounds was determined by the contraction of the muscles. FishBase lists a highly evolved capacity for hearing. Piranhas 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, and a science writer reporting on piranha experiments says that piranhas in larger shoals are far less stressed than those alone or in small shoals. You may have heard that piranhas hunt in packs. In fact, they also gather in groups for protection rather than hunting their prey in packs.

Piranhas also live in big groups called shoals and tend to travel in schools of 20 or more. Male piranhas build nests in the mud and sand at the bottom of the water. 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 stuck to plants growing under the water, and a female can lay thousands of eggs. A piranha can live about 10 years. It lives in the Amazon River basin and likes warm water, about 25 degrees Celsius, and the National Zoo says it is relatively common throughout its range, as of November 2026.

Words to know

• genome: all its genes
• piscivore: fish eater
• pitch: sound height
• opportunistic: taking what comes
• sequenced: decoded