What Do Piranhas Eat? Diet of the Red-Bellied Piranha

What does the red-bellied piranha eat?

The red-bellied piranha is an opportunist that takes almost any food it can bite. Studies of stomach contents and underwater observations in Brazil have recorded a wide menu:

Fish are usually the largest part of an adult’s diet, but the mix varies from place to place. In some lakes, plant matter and insects make up a surprising share, especially in the rainy season.

How the diet changes as piranhas grow

Newly hatched piranhas start on zooplankton, the tiny animals drifting in the water. As they grow, they move on to small crustaceans and aquatic insects, which they pick from water plants.

Juveniles and young adults are also notorious fin and scale biters. They dart in, take a quick nip from the fins or flank of another fish and retreat. Because fins regrow, a single victim can provide many small meals, and the biter takes little risk. This habit is most common in young fish and becomes less important as piranhas get bigger.

Adults rely more on fish. Their larger mouths and strong jaws let them remove a neat, crescent-shaped bite from a larger fish or swallow small fish whole. Large adults also scavenge more, eating dead fish and other carcasses.

Rainy season and dry season diets

South American rivers rise and fall by several meters with the seasons, and the piranha’s diet follows the water. In the rainy season, rivers spill into forests and grasslands. Fish, insects and fallen fruit and seeds are spread over a huge area, and piranhas eat more insects and plant matter while food is plentiful.

In the dry season, the water drains back into channels, lakes and isolated pools. Fish become crowded together, and food can run short. Piranhas then feed more on trapped fish and on carcasses, and competition within shoals becomes fierce. This is also the time when bites on swimmers are most likely, as explained in Are piranhas dangerous?

How piranhas find and catch food

Red-bellied piranhas use several feeding methods. They often lurk among water plants or roots and dash out to grab or bite passing fish. They may also chase prey in open water, pick insects and snails from vegetation, or scavenge whatever they find. Smaller fish, about 8–11 cm (3–4 in) long, feed mainly by day, while larger fish of about 15–24 cm (6–9.5 in) forage mostly at dawn, in the late afternoon and into the night.

Their teeth are ideal for this. Each jaw has a single row of flat, triangular, interlocking teeth that work like the blades of scissors, so a piranha can slice off a piece of food rather than having to swallow prey whole. Piranhas also have one of the strongest bites of any bony fish for their size: the black piranha’s bite has been measured at more than 320 newtons, about 30 times its body weight.

Piranhas are sensitive to splashing and vibrations in the water, which helps them find struggling or injured animals quickly. Several fish may then feed on the same item at once, which is where the image of a “feeding frenzy” comes from.

Do piranhas eat mammals and birds?

Piranhas do sometimes feed on mammals, birds and reptiles, but mostly as carrion or when an animal is already weak, injured or trapped. Groups of piranhas can quickly strip a carcass in the water, and small animals that fall into the water and struggle may be attacked.

Healthy large animals, such as capybaras, cattle and horses, cross piranha rivers all the time. The capybara, for example, spends much of its life in water shared with red-bellied piranhas. Piranhas also eat other piranhas: injured or dead shoal mates are eaten, and larger fish may take smaller ones.

Plant-eating piranhas and relatives

Not every piranha is a meat lover. In a study of more than 250 black piranhas from the Ji-Paraná River in Brazil, fish made up about 75–81 percent of the stomach contents, depending on the season, while about 10 percent was fruits or seeds. In a smaller study of another species from flooded forests on the same river, chewed seeds were the main food in many of the fish examined.

Some relatives in the same family go further still. Pacus, including the tambaqui (Colossoma macropomum), have blunt, molar-like teeth for crushing nuts and seeds, and they help spread the seeds of flooded-forest trees. A pacu-like fish named Tometes camunani, described in 2013 from rapids in northern Brazil, feeds mainly on riverweeds. At the other extreme, the wimple piranha (Catoprion mento) specializes in eating scales from other fish.

Why piranhas matter in the river

By eating dead and dying animals, piranhas help clean up rivers and recycle nutrients, a role often compared with that of vultures on land. They also keep populations of small fish and insects in check and are themselves eaten by caimans, river dolphins, giant otters, large fish and many waterbirds. Far from being simple killing machines, piranhas are flexible, opportunistic feeders that fit their diet to a constantly changing river.