Piranhas not strictly carnivorous

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A new study of the feeding behaviour of wild piranhas has shown that their fearsome reputation as killers might be exaggerated.

Rather than simply eating the flesh of other animals, new research has shown that two species of piranha are, in fact, generalist feeders that eat vegetable matter as well as fish.

The study, which has just been announced in scientific journal Acta Amazonica, was undertaken by Piorski, Alves, Machado and Correia and examined the feeding behaviour of Serrasalmus aff. brandtii and Pygocentrus nattereri.

Both species of piranha are common in Viana Lake, which is formed during the wet season when the Rio Pindare (a left bank tributary of the Rio Mearim) floods.

The researchers studied the feeding behaviour of the fish in the lake and looked at the qualitative composition of the diet using a frequency occurrence method.

They then caught piranhas three times each month using gill nets of varying sizes, to allow them to sample fish of different ages, and then examined the foods they had been eating by looking at the contents of their guts.

While the results show that both species are fish eaters, they also eat plant material, with P. nattereri being the bigger plant eater of the two species.

Things also change as the fish get larger: "Fish and plant material showed different relationships in relation to piranhas's length classes.

"The graphic analysis of the feeding strategies employed by P. nattereri and S. aff. brandtii suggest a generalist habit, wide width niche with strong participation of the within-phenotype component, which indicated that the majority of individuals of these species use several resources simultaneously.

"A multivariate analysis of the ecomorphological index indicated that the species are discriminated by swimming ability, water column position and relative prey size."

For more information see the paper: Piorski, NM, JRL Alves, MRB Machado and MMF Correia (2005) - Feeding and ecomorphology of two species of piranhas (Characiformes: Characidae) from the Viana Lake, Maranhao state, Brazil. Acta Amazonica 35: 63-70.