Their diet consists largely of fish, insects, worms, crustaceans, and the occasional larger animal. In contrast to their reputation, they usually feed on dead, dying, and injured vertebrates in the wild, but have been known to attack healthy animals. The fish usually feed in large schools around dusk and dawn. They locate their prey by scent or motion using a set of sensors down the sides of their bodies, the lateral line system.
Red-bellied piranha do spawn around April and May during the rainy season. The male will build a dug-out nest in rocks and vegetation, awaiting a female. Females can lay around 600 eggs which the male fertilizes. Males become extremely territorial during spawning, and will prevent other fish from approaching the nest. After the eggs hatch, both parents guard the broods. Red-bellied piranhas exhibit very little obvious sexual dimorphism, except females may have slightly more yellow on the belly than males.
Red-bellied piranha in media
Many myths surround this species. The 1978 film Piranha by Joe Dante shows these fish in a similar light to Jaws. Piranha was followed by a sequel, Piranha II: The Spawning, in 1981, and two remakes, one in 1995, and one in 2010. Films such as these, and stories of large schools of red-bellies attacking humans, fuels their reputation as being one of the most ferocious of all freshwater fish. In reality, they are generally shy or also known as timid scavengers, fulfilling a role similar to flocks of vultures on land. In the 2010 film Piranha 3D, Christopher Lloyd's character identifies a specimen of the fictional monstrous piranha, specifically as Pygocentrus nattereri, but erroneously refers to them as the first piranhas, when in reality, red-bellied piranha are most likely not the "original" species.
In an aquarium
Despite their fearsome reputation, piranhas can be kept as aquarium fish. Their diet in nature consists of live prey and dead animals and fish. Live feedings to captive piranhas are not advised, as they can introduce diseases, and goldfish contain a growth-inhibiting hormone which in turn will affect piranhas. Many experienced piranha keepers feed their fish on lean meat, such as beef heart, unbreaded fillets of common table fish, and invertebrates, such as shrimp and crab. They may be kept with certain armoured catfish, such as Hypostomus plecostomus, but this is generally discouraged amongst the piranha-keeping community. Some have reported success keeping them in large shoals with other large fish in aquariums upwards of 1300 liters (300 US gallons), but any introductions of other species of fish should be done with extreme caution. Red-bellied piranhas should be kept in small shoals of at least four individuals, to encourage dispersal of aggression, or individually, which is common practice. These fish, particularly when juvenile, will sometimes bite one another in the aquarium, normally on the fins, in behaviour called 'fin nipping'. Fish that have had their fins nipped will grow them back surprisingly rapidly. These fish can be timid in the aquarium, in contrast to their reputation. This can be for a variety of reasons, i.e. due to unnaturally high light conditions, poor water quality, and lack of cover, which juvenile fish in particular need to hide. Some hobbyists have been disappointed with the fish's timidity in the aquarium, having acquirred them for their exaggerated reputations as killers. Piranhas require frequent water changes with soft water. Extensive filtration is required, and the turnover of the filter should be at least three times the aquarium capacity per hour, as they generate a lot of waste owing to their diet and because they usually cannot be kept with scavenger fishes in their tank. A tank containing red-bellied piranha should be kept at a constant pH of 5.5 to 7.0, and water parameters of nitrate, nitrite, and ammonia monitored at least every week. Furnishing the aquarium with bogwood (mangrove root, e.g.) has been suggested to assist in keeping the pH constant.