a single red bellied piranha

Which Piranha Is The Most Aggressive?

With 30 to 60 species of Piranha living in the Amazon basin, a lot of the time they spend their time scavenging and foraging. Although they have a taste for meat, are all Piranha dangerous?

So, which Piranha is the most aggressive? In short, the Red-Bellied Piranha is considered the most dangerous and aggressive of all the Piranha. It is also the most common in the wild, and the most desired for an aquarium. While it scavenges and forages most of the time it still can be an aggressive fish. During the dry season it can become very aggressive defending a nest or dealing with extreme hunger. It will happily take part in feeding frenzies on dead meat, making the water look alive fish. It will naturally be shy and timid if it suspects it is in the presence of a larger animal, fearing it may be a predator.

The Red-Bellied Piranha

With the species being named for its teeth (The Brazlian language of the Tupi people named it Piranha or ‘tooth fish’), you might suspect that this is a ferocious fish.

With the Latin name of Pygocentrus nattereri, the Red-Bellied Piranha is indeginous to Latin America and thrives in all the rivers, wetlands, floodplains and reservoirs in the Amazon basin.

It has an omnivorous nature and spends it time foraging and scavenging for food, feeding on insects, worms, crustaceans, fruits, seeds, nuts and other fish.

Being part of the pygocentrus genus, which characterizes the Piranha as preferring school like behaviour which experts have determined is more defense rather than an attacking position.

It has the reputation of a ferocious freshwater predator, a bloodthirsty and mindless killer but in reality it is a shy and retiring fish. They only attack humans when they are threatened or with extreme hunger, more common in the dry season.

Their reputation seems to have stemmed from a former US president, President Theodore Roosevelt. Writing for his book ‘Through the Brazilian Wilderness’ he was treated to a spectacle where he wrote that the Piranha was the “most ferocious fish in the world.”

a single red bellied piranha

Further, he wrote

“The piranhas habitually attack things much larger than themselves,” Roosevelt wrote. “They will snap a finger off a hand incautiously trailed in the water; they mutilate swimmers—in every river town in Paraguay there are men who have been thus mutilated; they will rend and devour alive any wounded man or beast; for blood in the water excites them to madness. They will tear wounded wild fowl to pieces, and bite off the tails of big fish as they grow exhausted when fighting after being hooked.”

While the entire episode with a cow might have been staged to provide a dazzling display for the famous guest of the country, it certainly did cement the reputation of the fish.

As it has this reputation, it turns out to be a popular aquarium choice for any prospective Piranha owner.

Are All Piranha Aggressive?

Despite the Piranhas almost unanimous reputation of a mindless predator, it should be noted that there are 30 to 60 species, and not all of them have a taste for meat.

Some are vegetarian.

While the films and media make all Piranhas sound like they will only eat meat, the truth is there are specifically Piranha that are designated as herbivores. The Tometes camunani Piranha is one such fish. They are not aggressive towards humans.

Also, when the wet season arrives, a flood of water covers the amazon basin, Hude reservoirs and wetlands are formed and the rivers swell. During this time, the fruit trees bear fruit and Piranha take the fallen fruit as a meal. With a plentiful food supply of natural food, Piranhas will be less aggressive.

4 red bellied piranha in an aquarium

A Warning Of Aggressive Behavior

As strange as it might seem for this ‘mindless killer’, the Red-Bellied Piranha have been recorded making three very distinct sounds, which have been described as the Piranha barking a warning, much like a dog.

It seems to be an inflation and a contraction of the swim bladder, and a gnashing of teeth. The sounds resemble a grunt, a bark and a click. It’s is an increasing escalation of sounds that signifies an increasing level of annoyance. They are used to warn other fish, or other Piranha to ‘back off’.

Piranha face on

What Prompts Aggression

Some things however, will prompt aggression. The Red-Bellied Piranha is not a defenceless fish, possessing razor sharp interlocking teeth and a bite that is one of the most powerful in the animal kingdom.

The Red-Bellied Piranha is instinctively programmed to respond to noise, splashing, and blood in the water. A school of Red-Bellies are said to send out a scout to see what the food source may be.

If the animal is large, then the Piranha will shy away and retire to cover, They are very aware that the river dolphins, caimans and crocodiles of the Amazon basin are looking for their next Piranha meal.

However, if the animal is assessed to be dead, then Piranha can attack in a school of fish. This behaviour is said to be the Piranha frenzy.

With small fish, that are exhibiting signs of distress or injury, a Piranha school will attack quite ferociously, with each taking a bite and turning away quickly to let another feast.

In an aquarium, there are many things that can trigger aggression, from space in the tank to lighting being too strong. Or indeed, too many people walking past the tank. All of which can produce stress in the piranha and aggravate them

The Dry Season

While the wet season is under full swing, Red-Bellied Piranha are well fed and not a great danger to humans around them.

However, during the dry season a couple of things happen which makes them much more aggressive in nature. Piranha are notoriously known for biting more people during this season.

Firstly, the end of the dry season is when the Piranha breed.  A pair of Red-Bellied Piranha will swim off, create a small nest in the river bottom, near foliage and breed, The female will lay up to 5000 eggs, which are then guarded by the pair while they hatch, which is normally 2 to 3 days. During this time they are ferocious with intruders and will routinely bite and snap at other fish wandering by.

Interestingly, this is also the time when most humans get bitten. Bathers in the water often wade near a Piranha nest and the Piranha rather instinctively bites a foot or a finger to scare you away. Something that it is quite successful of doing if you read the media reports.

Secondly, as the water recedes so do a lot of the food sources. Towards the end of the dry season, formerly large wetlands will become small pools. The same number of Piranha in them but less food. With extreme hunger and a lack of space they are a lot more aggressive. 

Amazon river
The Amazon river: 70 – 88 degrees Fahrenheit


The media has given the Piranha a bad reputation. From James Bond films to media reports they are often mistaken for mindless killers. While this isn’t the whole truth, as they can certainly behave that way, there are caveats to that behavior.

Normally Red-Bellied Piranha are rather shy and timid but can become a lot more aggressive when facing extreme hunger of defending nests. They are notoriously aggressive during this period.

In an aquarium, a Red-Bellied Piranha facing stress, likewise will tend more towards aggression. From the lack of space, to too many bright light with not enough decoration for them to hide may produce aggressive fish.

So, while not a fish that is attacking things every minute it is not defenseless either. The Red-Bellied Piranha will bark at its aggressor, in an increasingly harsh set of warnings before it attacks. If that aggressor does not heed the warning, it had better watch out because they are quick and powerful fish, with razor sharp teeth.


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