The red-bellied piranha ( Pygocentrus nattereri ) is a freshwater ray-finned fish belonging to the family Serrasalmidae, which includes pacus, piranhas, and related fishes. Native to South America, specifically the Amazon, Paraguay, Paraná, and Essequibo basins, as well as coastal rivers of northeastern Brazil, they are locally abundant in their freshwater habitat. Often misrepresented as ferocious predators, red-bellied piranhas are primarily omnivorous foragers and scavengers.
Physical Characteristics
Red-bellied piranhas are gray with pronounced black spots at birth and develop their signature reddish tinge to the belly when fully grown. Juveniles are a silver color with darker spots. Adult piranhas can reach up to 50 cm (20 in) in standard length and 3.9 kg (8 lb 10 oz) in weight, but rarely surpass 35 cm (14 in). The rest of the body is often grey 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. They are characterized by deep, lateral compressed bodies and long dorsal fins. Within the family, red-bellied piranhas are classified in the genus Pygocentrus, which is distinguished by the unusual dentition and differing head width dimensions. Beneath the high forehead are powerful muscles that attach to a short, stout lower jaw fitted with triangular, razor sharp teeth, which interlock neatly with a matching set above. This design allows piranhas to bite down with incredible force and shearing ability.
Habitat and Distribution
Pygocentrus nattereri encompasses a larger geographic area than any other piranha species, covering much of the Neotropical region. The red-bellied piranha is widely distributed throughout the South American continent and is found in the Neotropical rivers of Argentina, Brazil, Bolivia, Colombia, Ecuador, Guyana, Paraguay, Peru, Uruguay, and Venezuela. They live in the warm freshwater drainages of several major rivers including the Amazon, Paraguay, Paraná and Essequibo, as well as numerous smaller systems. They are 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 can live in waters that are between 15 and 35 °C (59-95 °F), but are able to survive temperatures as low as 10 °C (50 °F) for a period. They are mainly found in whitewater, but have also been recorded in blackwater and clearwater.
Social Behavior
Red-bellied piranhas often travel in large groups called shoals. Typically, the younger piranhas are found toward the outside of the group and the larger, sexually mature adults are the center. This placement protects the adults from predators while giving the young easy, constant access to food. This shoaling behavior is a predatory defense against larger animals such as dolphins, large piscivorous fish, caimans, and aquatic birds such as storks, herons, and anhingas. They live in shoals but do not group hunt, 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.
Diet in the Wild
The typical diet of red-bellied piranhas is omnivorous including fruit, leaves, insects, mollusks, carrion and fish. They are primarily foragers and feed on insects, fish, plants, and organic debris. They are opportunistic feeders as they are, will also take advantage of edible aquatic plants as well as any fruits, nuts or seeds that may fall into the waters. In packs up to hundreds, piranhas have been known to feed on animals as large as egrets or capybara. Despite the piranha’s reputation as a dangerous carnivore, it is actually primarily a scavenger and forager, 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.
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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.
Reproduction
Breeding occurs over a two-month period during the rainy season, but that can vary by area. The breeding habits of piranhas in nature are mostly unknown, with most spawning research being done in aquariums. 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. 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. 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 toward other fish that approach 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. They are not a migratory species but do travel to seek out conditions conducive to breeding and spawning during periods of increased rainfall.
Communication
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. All of the observations made on sound production by red-bellied piranha 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. Nearly all sounds produced by red-bellied piranhas are produced in the context of social interactions between individuals.
Misconceptions and Reality
The red-bellied piranha has a popular reputation as a ferocious predator, despite being primarily a scavenger. A significant amount of lore regarding the ferocity of the red-bellied piranha can be traced to President Theodore Roosevelt following the completion of the Roosevelt-Rondon Scientific Expedition. The former president returned with dramatic stories of an entire cow being devoured to the bones within a matter of a few minutes. This event was later found to be staged by local guides, eager to provide Roosevelt with a spectacle worthy of the journey. It later surfaced the fish had been corralled and starved for over a week, into a section of the river, into which the unfortunate cow was driven for the event. Films such as these, and stories of large schools of red-bellies attacking humans, fuel 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.
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