Present-day species of piranha result from a marine incursion into the Am...( Piranhas inhabit exclusively the fre...)

Piranhas inhabit exclusively the fresh waters of South America. Their geographical distribution extends from the Orinoco River basin (Venezuela) to the North, down to that of the Paran (Argentina) to the South. Over this whole area, which also embraces the entire Amazon Basin, biologists have recorded 28 carnivorous species of these fish (2). In spite of the evolutionary success of this subfamily of fish, the mechanisms that generated the species richness of this group are still insufficiently known.

A team from the IRD, working in partnership with Bolivian and Peruvian scientists, aimed to establish how these species were able to evolve over the past 15 million years. They consequently took samples from around their whole distribution range. Between September 2002 and June 2003, numerous specimens of piranhas were collected from the Bolivian part of the Amazon. Complementary sampling was then conducted in the Brazilian and Peruvian sectors, from the Orinoco in Venezuela, and the So Francisco and the Paran-Paraguay in Brazil. The team selected 57 specimens representative of 21 different species of piranhas, from 15 collection points distributed over the whole South-American hydrographic network,.

Mitochondrial DNA (mtDNA) of piranhas has a particularly high mutation rate and thus could be used as a molecular basis for reconstructing the evolution of the present-day species which are different yet very close to one another. These techniques using mtDNA sequences led to the conclusion that the origin of the piranha species inhabiting the rivers of South America today dates back to some ancestor at only a few million years B.P. Yet dating from fossils, whose morphologies are strikingly similar to those of present-day piranhas, strongly suggests that this fish subfamily already existed in South Americas hydrographic system 25 million years ago. The modern species must therefore stem from a recent diversification.

Further investigation involving the c
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