Scientists have sequenced and assembled the red-bellied piranha genome. The genome was predicted to hold 25,861 protein-coding genes, the genes that carry the plans for making proteins. In the same study, the fish matured at about 16 cm total length. The species is a fish that lives in rivers in South America, in the Amazon River basin. At birth, the fish are gray with pronounced black spots, and an adult has a red belly and a body flecked with silver. 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. It also eats snails and other mollusks, and carrion, and it is a scavenger in the Amazon basin.

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, and only the head portion of the swim bladder contributed to sound production. 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 says that piranhas in larger shoals are far less stressed than those alone or in small shoals. A shoal can have more than a hundred fish in it. In a shoal, the younger piranhas are often toward the outside of the group and the larger, sexually mature adults are in the center.

A river study of piranhas identified 326 fish of nine species and three genera. The study took samples monthly from August 2020 to July 2021 and found differences between high-water and low-water periods. In that study, 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. One study suggests the plant intake of the red-bellied piranha may be an active way of gaining food supplies while scanning for prey. In the wild, a good portion of its diet is fins nipped from the tails of larger fish.

Piranhas also live in big groups called shoals, they gather in shoals for protection, and they tend to travel in schools of 20 or more. In the wild, male piranhas build nests in the mud and sand at the bottom of the water. Spawning seems to occur during the wet season, and in the wild piranhas show two annual reproductive seasons. 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, and the eggs are laid in clusters and stuck to plants growing under the water. A female can lay thousands of eggs, and a piranha can live about 10 years. The National Zoo says the species is relatively common throughout its range, as of November 2026.

Words to know

• genome: all its genes
• genera: species groups
• bladder: a body sac
• shoals: fish groups
• pitch: sound height