The species are both members of the family Serrasalmidae and both are found throughout the Amazon basin. They have very different diets, however, and the largely vegetarian pacu benefits from being confused with one of the most notorious carnivorous fish in freshwater.
Batesian mimicry was first proposed by naturalist Henry Walter Bates in the 19th century and describes a harmless species that adopts the signalling of a harmful species in order to deter predators.
In the instance of the red-bellied pacu, or pirapitinga as it is also known, a resemblance to the piranha will prompt other species to keep their distance for fear of being torn to shreds. Both are round-bodied, with dull-grey or green bodies and a flush of blood-red colouring across the belly.
While the piranha has needle-sharp teeth, built for tearing and stripping flesh, those of a pacu are similar to human molars in appearance. The flattened surface of the teeth is ideal for grinding nuts, seeds and fruit upon which the pacu predominantly feeds.
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The pacu can reach 25 kg in weight, at which point, with fewer potential predators, mimicry of the piranha is no longer as important, so the red of the belly usually fades.
Their wider distribution has furthered an erroneous belief that the pacu will deliberately bite a man’s testicles. Rumours have circulated for some years suggesting that swimmers, naked in the water, have been castrated or even bled to death.
Pacu are a popular food fish, with overfishing a potential issue across their native range. Elsewhere in the world, they have become popular in aquaculture, and feral, invasive colonies have become established in the south-eastern US.
Perhaps the pacu’s fondness for eating nuts has been misconstrued…
