Piranhas have a reputation as some of the most dangerous freshwater hunters: terrifying predators capable of devouring a large mammal in mere minutes.
It turns out, however, that Hollywood has a lot to answer for. While many of the more than 60 species that make up the Serrasalmid family are indeed carnivorous, piranhas are mainly opportunistic, scavenging more often than they kill.
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How sharp are piranha teeth?
The piranha's most recognisable feature is their triangular, razor-sharp teeth. The red-bellied piranha (Pygocentrus nattereri) is the most infamous, boasting the strongest jaws and sharpest teeth, and growing up to 50cm long. Piranha teeth are adapted for a carnivorous diet, interlocking in a scissor-like bite.
Their enamel structure is similar to that of sharks, but unlike sharks, which replace teeth individually, piranhas replace their teeth in blocks, losing all the teeth in the lower left jaw, say, when the ones beneath are ready to erupt. While very obvious in scary pictures of the fish, however, these teeth are usually covered up by thick lips.
How powerful is a piranha bite?
The black (or redeye) piranha (Serrasalmus rhombeus) holds the record for the strongest bite of any bony fish, three times greater than the bite force of an American alligator of equivalent size.
How big are piranhas?
Piranhas range from around 30cm to around 60cm. Their teeth are usually around 4mm tall.
What do they eat?
Despite their reputation, piranhas are more likely to scavenge than kill. Piranhas are opportunistic feeders. While some species hunt, most scavenge or consume plant material. Their diet includes insects, fish, crustaceans, worms, carrion, seeds, fruit and plant material.
Certain piranhas are omnivorous, with some species relying primarily on plants. Tometes camunani, for example, which is found in Brazil’s Trombetas basin, feeds exclusively on river weeds. Wimple piranhas (Catoprion) survive by harmlessly nipping fins and scales from living fish. The availability of food also affects piranhas’ diet: in scarce conditions, some species may resort to cannibalism. A red-bellied piranha eats about 2.46 grams per day – roughly one-eighth of its body weight.
How do piranhas hunt?
Red-bellied piranhas, known for their group hunting behaviour, typically target prey their own size or smaller. Piranhas have an acute sense of smell, detecting a single drop of blood in 200 litres of water. A scout locates the prey and signals the rest, likely using sound. Shoals tend to number at least 20 individuals, up to around 100. Their feeding frenzies, while dramatic, are rare and usually triggered by extreme hunger or distress. The fish attack in rapid succession, each one taking a bite before making way for the next.
Smaller fish hunt or forage for food by day, while larger specimens tend to be more active at dawn and dusk.
Are piranhas sociable?
Red-bellied piranhas are known for forming shoals, but not necessarily with hunting in mind. Instead, shoaling provides protection. Larger, older fish typically stay at the centre, while younger fish occupy the outer edges, reducing the risk to mature individuals.
How dangerous are piranhas?
With their sharp teeth and strong bite force, piranhas are formidable but not as deadly as popular opinion might suggest. While capable of inflicting serious, painful bites, attacks on humans are rare. Most injuries occur when people unknowingly disturb nests. The majority of bites involve a single wound rather than coordinated attacks. Reports of piranhas consuming human bodies typically involve scavenging – after a person has drowned, for example.
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The most aggressive species toward humans include the red-bellied piranha, the lobetoothed piranha (P. denticulata) found in the Orinoco River, and the San Francisco piranha (P. piraya) of Brazil’s San Francisco River. Attacks are more likely when water levels are low, food is scarce, or nests are disturbed. Fishermen handling nets and hooks are at greater risk than swimmers. If bitten, it’s typically a single bite, often targeting toes or feet.
Where do piranhas live?
Piranhas can be found in rivers and lakes across most of South America, from northern Argentina to Colombia, with 20 species in the Amazon river alone. Fossils suggest their ancestors appeared in South American rivers some 25 million years ago, though modern species have only existed for about 1.8 million years.
In very much more recent times, piranhas have spread elsewhere around the world too, thanks to pet owners releasing them into non-native water. They’ve been spotted in the UK, China and America, where they can pose a threat to native species.
How do piranhas mate?
Red-bellied piranhas engage in a courtship display that sees them swim in circles. The female will then lay eggs in clusters into a nest dug into the riverbed by the male. The male guards the eggs aggressively until they hatch.
What noises do piranhas make?
Red-bellied piranhas produce distinct sounds for communication. They emit bark-like noises when caught by fishermen. Scientists have identified three vocalisations: quick barks during confrontations, low grunts or thuds while circling or fighting, and percussive clicks. They generate these sounds by contracting muscles around their swimbladder, an organ used for buoyancy.
What preys on piranhas?
Despite their fearsome reputation, piranhas are not apex predators. Their group behaviour and sharp teeth provide some defence, but they remain vulnerable. They are preyed upon by caimans, birds, river dolphins and larger carnivorous fish, as well as humans – caldo de piranha is a traditional soup from the Pantanal area of Brazil.
Special features of piranhas:
Scientific family name: Serrasalmid
Length: 30cm-60cm, depending on species
Weight: 0.9kg-3kg, depending on species
Average lifespan: 10 years
Annotations: Blunt head, strong jaws, triangular, razor-sharp teeth, thick lips, vary in colour from silvery with orange undersides to nearly black
Main image: Close up of the piranha genus Serrasalmus/Getty
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