Sources describe the diet of red-bellied piranhas differently. A genome paper says the species is mostly a carnivorous piscivore, a fish eater, that can be opportunistic and feed on plants, insects, worms and crustaceans, while a hobbyist profile says piranhas of the group called Pygocentrus are more accurately described as opportunistic generalists. Biologist Steve Huskey of Western Kentucky University says piranhas really do not eat things other than fins, which is a different picture from the wide diets in other sources. A Western Kentucky University team argued that piranhas eat mostly other fish fins, not giant fish that could easily swallow them whole. In the wild, a good portion of the red-bellied piranha's diet is fins nipped from the tails of larger fish, and the species mostly ambushes fish from within the aquatic vegetation and dashes after passing prey.

In a 2020-2021 study of piranhas in a Brazilian river, fish remains were the most relevant food items at all the collection points. The study identified 326 fish of nine species and three genera, and it took samples monthly from August 2020 to July 2021, finding differences between high-water and low-water periods. The stomach contents of 56 piranhas were analysed, and 55.35% of the stomachs were full and 44.65% were empty. One study suggests the plant intake of the red-bellied piranha may be an active way of gaining food supplies while scanning for prey, and the species is described as a scavenger in the Amazon basin that also eats snails and other mollusks, carrion and fruit.

Scientists have sequenced and assembled the red-bellied piranha genome, the full set of its genes. The genome was predicted to hold 25,861 protein-coding genes, the genes that carry the plans for making proteins. In the same genome study, the fish matured at about 16 cm total length. That study is where the description of the species as mostly a fish eater comes from, and it sits beside the river study and the university team's work as one more piece of evidence about what these fish do.

Red-bellied 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. In red-bellied piranhas, only the head portion of the swim bladder contributed to sound production, and FishBase lists a highly evolved capacity for hearing.

Red-bellied 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. In a shoal, the younger piranhas are typically toward the outside of the group and the larger, sexually mature adults are in the center.

Male red-bellied piranhas build nests in the mud and sand at the bottom of the water, and a nest is around 4 to 5 cm deep and 15 cm across. Spawning seems to occur during the wet season, and in the wild red-bellied piranhas show two annual reproductive seasons. A courtship starts with swimming around in circles, and 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. The species lives in the Amazon River basin, the Paraguay-Paraná River basin and rivers of northeastern Brazil, often in whitewater streams and often not in blackwater streams, and the National Zoo says it is relatively common throughout its range, as of November 2026.

Words to know

• genome: all its genes
• generalists: wide eaters
• analysed: studied
• piscivore: fish eater
• spawn: fish eggs
• shoals: fish groups