Flooding triggered piranha evolution

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Biologists have discovered that present day species of piranha have evolved as a result of the Atlantic ocean flooding the Amazon plain five million years ago.

A team from the Institut de recherch pour le Dveloppement (IRD) together with researchers from Bolivia and Peru showed that although fossil records for piranha species similar to present day species existed 25 million years ago, modern piranha species are actually only around three million years old.

Piranhas are endemic to the Amazon basin. They are omnivorous and play a key part in the ecology of these rivers eating fish plants and insects but also acting as prey for dolphins, caiman and larger fish.

The mitochondrial DNA (mtDNA) of piranhas has an extremely high mutation rate so this was used as a basis for reconstructing the evolution of the 28 modern species of this carnivorous fish.

By constructing a phylogenetic tree of the evolutionary relationship between the living species and comparing it a geological time scale the team established that the advancement of the Atlantic Ocean onto the Amazon flood plain five million years ago played a key role in the appearance and then the distribution of piranha species.

The invasion of saline water up to altitudes of 100 metres meant that many existing freshwater species disappeared and yet more (including some species of riverine dolphins) were introduced.

This theory suggests that some ancient piranha species would have survived at the upper reaches of the Amazon and would have differentiated into species as the water retreated and their habitat fragmented and also in response to ecological constraints such as water quality unique to their habitat.

Once the ocean regressed three million years ago, the piranhas would have dispersed back downstream to the Amazon s lowland plain.

For more information see: Institut de Recherche Pour le Dveloppement. "Present-day Species Of Piranha Resulted From Marine Incursion Into Amazon Basin." ScienceDaily 3 December 2007. 4 December 2007