The Red-bellied piranha (Pygocentrus nattereri) is a species of piranha. This species lives in the Amazon River Basin, coastal rivers of north-eastern Brazil, and the basin of the Paraguay, Paraná and Essequibo Rivers. The red-bellied piranha has the reputation for being one of the most ferocious freshwater fish in the world. They have a set of razor sharp teeth which are capable of stripping flesh from prey and dead animals. As their name suggests, they have a reddish tinge to the belly when fully grown, although juveniles are a silver colour with darker spots. They grow to a maximum length of 33 cm and a weight of 3.5 kg.
Their diet consists, for the most part, of fish, insects, worms and crustaceans but may sometimes eat large animals. In contrast to their reputation, they primarily feed on dead, dying, and injured vertebrates in the wild, but have been known to attack healthy animals on occasion. 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 the their bodies, called the lateral line system.
Red-bellied piranha usually 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 brood. Red-belly piranhas exhibit very little obvious sexual dimorphism, except that females may have slightly more yellow on the belly than males.
Red-bellied piranha in media
There are many myths surrounding 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 another which is forthcoming. Films such as these, and stories of large shoals of red bellies attacking humans, fuels the reputation of the red bellied piranha as being one of the most ferocious of all freshwater fish. In reality, they are generally timid scavengers, fulfilling a role similar to flocks of Vultures on land.
In the aquarium
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Piranha, despite their fearsome reputation, can be kept as aquarium fish. Their diet in nature consists of live prey and dead animals and fish. Live feedings to captive piranha are not advised, as they can introduce diseases, and goldfish contain growth-inhibiting hormone which in turn will affect piranha. Many experienced piranha keepers feed their fish on lean, chemical-free 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 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 on their own which is common practice. These fish, particularly when juvenile, will sometimes bite one another in the aquarium, normally on the fins, in behaviour not surprisingly 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, getting them due to their exaggerated reputations as killers. Piranha requires frequent water changes, with soft water. Extensive filtration is required and the turnover of the filter should be at least 3 times the aquarium capacity per hour, as piranha generate a lot of waste owing to their diet. The tank 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 etc.) will assist in keeping the pH constant.