The red-bellied piranha or red piranha (Pygocentrus nattereri) is a species of piranha native to South America, found in the Amazon River Basin, coastal rivers of northeastern Brazil, and the basins of the Paraguay and Paraná. They are omnivorous foragers and feed on insects, worms, crustaceans and fish. They are not a migratory species, but do travel to seek out conditions conducive to breeding and spawning during periods of increased rainfall. Red-bellied piranhas often travel in shoals as a predatory defense, but rarely exhibit group hunting behavior. Acoustic communication in common, and is sometimes exhibited along with aggressive behaviors. At this time, the red-bellied piranha is not considered to be a threatened species by the IUCN, and therefore, there are no conservation strategies in place to target this species. Through media influence, the red-bellied piranha has developed a reputation as a ferocious predator, though this is not actually the case. They are a popular aquarium fish.
The red-bellied piranha has a popular reputation as a ferocious predator, despite being primarily scavengers. As their name suggests, red-bellied piranhas 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 centimetres (13 in) and a weight of 3.5 kilograms (7.7 lb). The rest of the body is often gray with silver-flecked scales. Sometimes, blackish spots appear behind the gills and the anal fin is usually black at the base. The pectoral and pelvic fins may vary from red to orange. Females can be distinguished from males by the slightly deeper red color of their bellies.
The red-bellied piranha is typically found in white water rivers, such as the Amazon River Basin, and in some streams and lakes. Sometimes, they may inhabit flooded forests such as those found in the Brazilian Amazon. They live in shoals but do not do group hunting behavior, although they may occasionally enter into feeding frenzies. In the case of a feeding frenzy, schools of piranha will converge on one large prey individual, and eat it within minutes. These attacks are usually extremely rare and are due to provocation or starvation. Breeding occurs over a two month period during the rainy season, but that can vary by area. Females will lay around 5,000 eggs on newly submerged vegetation in nests that are built buy the males. 
Pygocentrus nattereri encompasses a larger geographic area than any other piranha species, covering much of the Neotropical region. When red-bellied piranhas are introduced to other parts of the American continent, there are usually negative consequences for the local fish fauna, partially due to its generally aggressive behavior. This aggressive behavior is sometimes marked by the acoustic sounds they produce.
The red-bellied piranha is not a migratory species, but does search for conditions conducive to reproduction during seasons of increased rainfall. Red-bellied piranhas are omnivores and primarily foragers. They feed on insects, fish, plants, and organic debris.
Diet and Feeding Behavior
The typical diet of red-bellied piranhas includes insects, worms, crustaceans, and fish. Despite the piranha’s reputation as a dangerous carnivore, it is actually primarily scavengers and foragers, and will mainly eat plants and insects during the rainy season when food is abundant. They also tend to only feed on weak, injured, dying, or dead animals in the wild. Red-bellied piranhas do not stay in groups in order to pack-hunt for larger animals, but instead group for protection against predators.
Foraging methods vary throughout the different stages of a piranha's life. Smaller fish will search for food during the day, while larger fish will forage at dawn, in the late afternoon, and in the early evening. Throughout the day, the fish lurk in dark areas and ambush their prey. The piranha may also catch prey by hunting and chasing, where it will lie hidden in the vegetation until its prey swims by. The piranha will then capture its prey. When scavenging, the piranha will eat a wide variety of food, ranging from pieces of debris, insects, snails, fish fins and scales, and plants.
The breeding habits of piranhas in nature are mostly unknown, with most spawning research being done in aquaria. Piranhas are usually able to breed by the time they are one year old. Female piranhas will lay several thousand eggs near water plants, onto which the eggs stick. The males then fertilize the eggs. After just two to three days the eggs will hatch, and the juvenile piranhas will hide in the plants until they are large enough to defend themselves.
Research on red-bellied piranha breeding behavior in nature has revealed certain behavioral patterns around nesting sites. Adult piranhas will swim side-by-side in small circles, sometimes with two individuals swimming in opposite directions while keeping their ventral surfaces close to one another. Although this may appear to be a courtship display, a closer look reveals that the adults are actually defending nesting sites. The nests are about 4 to 5 cm deep, and are dug amongst water grasses, with the eggs attached to the grasses and plant stems.
This formation of mating pairs, nuptial swimming displays, and guarding of the nests shows that red-bellied piranhas exhibit parental care for the nest and the young. When left unattended, other fish, such as characids, may prey upon the eggs. Despite the defensive practice of circling the nests, red-bellied piranhas are often passive towards other fish that approach the nest. It is possible that the mere presence of the piranha, a natural predator, provides enough of a threat to prevent potential predators from approaching the nest.
Piranhas have two annual reproductive seasons; these seasons are tied to water level fluctuations, the flooding pulse, temperature, and other hydrological conditions. When individuals are ready to become sexually active, they will lose their red coloration and select habitats that are conducive to spawning, such as flooded marginal grasses and vegetation within lakes. This habitat selection is a clear distinction from non-reproductive individuals that prefer open water and under floating meadows.
Red-bellied piranhas often travel in shoals as a predatory defense. In studies that tested the piranhas' reactions to a simulated predator attack, resting opercular rates returned to normal more quickly amongst piranhas that were in shoals of eight rather than in shoals of two. Although it has been presumed that piranhas engage in pack-hunting behavior, no investigation shows that shoaling behavior among piranhas is used for cooperative hunting.
Most likely, this shoaling behavior is a defense against predation from larger animals such as dolphins, large piscivorous fish, caimans, and aquatic birds. Piranhas will travel to their nesting sites in shoals in order to reduce the likelihood that any single individual will be attacked by a predator. Shoals of red-bellied piranha use the margins of flooded areas to build their nests.
Communication and signaling
Acoustic communication among red-bellied piranhas is exhibited along with aggressive behaviors, such as biting, chasing, conspecific confrontation, and fighting. The sounds created by piranhas are generated through rapid contractions of the sonic muscles and is associated with the swimbladder. The swimbladder may play an important role in sound production as a resonator.
Most of the observations made on sound production by red-bellied pirhana have been when specimens were held by hand. When taken out of the water, the red-bellied piranha will emit a drumming-like sound, consisting of a low-frequency harmonic sound. However, research has shown the presence of three types of acoustic emissions that are associated with specific behaviors. Type one calls are made up of harmonic sounds, last approximately 140 milliseconds at 120 Hz, and are associated with frontal display behavior between two fish. Type two sounds last approximately 36 milliseconds at 40 Hz, and are associated with circling and fighting behavior related to food competition. Type three sounds are made up of a single pulse lasting just 3 milliseconds at 1740 Hz, and are highly associated with chasing behavior toward a conspecific individual. This same sound is also produced when an individual snaps its jaws to bite another individual.
Nearly all sounds produced by red-bellied piranhas are produced in the context of social interactions between individuals. The low, drumming sounds are typically produced during moderate attacks, while loud, threatened sounds are produced during more vigorous attacks.
Currently, the red-bellied piranha is not classified as threatened or endangered by the International Union for Conservation of Nature. Though the species is not considered threatened, the collection and trade of the species to aquariums may be causing a low risk to the red-bellied piranha. At this time, there are no specific conservation efforts for the red-bellied piranha. 
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 exaggerated and erroneous reputation as being one of the most ferocious freshwater fish. In reality, they are generally timid scavengers, fulfilling a role similar to 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
Red-bellied piranhas are sometimes kept as aquarium fish. Their natural diet consists of live prey and dead animals and fish. Live feedings to captive piranhas can introduce diseases, and goldfish contain a growth-inhibiting hormone which in turn will affect piranhas. Red-bellied piranhas, 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. In order to maintain a piranha aquarium, it is important to keep the water quality up, as they are messy eaters, and this will dirty the water in the tank. They may be fed live, fresh, or frozen food types, but they will not eat rotten meats. Because piranhas in the wild may not eat every day, piranhas in captivity do not need to be fed daily. 
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