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Origin
of Piranha
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 especially 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 construction of a phylogenetic tree by
categorizing the studys 21 species allowed phylogenetic relationships between
each of them to be established in order to test alternative hypotheses for
the diversification that occurred over time. Examination of these data
alongside geological-scale changes that have affected aquatic ecosystems with
time brought out evidence that marine incursions played a fundamental role in
the appearance then the distribution of piranha species. Five million years
ago, the Atlantic Ocean advanced, its waters finding their way far onto the
Amazon flood plain. The saline water invaded the lowland expanse of the great
river and penetrated its tributaries situated below 100 metres of altitude,
provoking the disappearance of a number of species of freshwater fish. Some
of these would nevertheless have succeeded in finding refuge at high
altitude, in particular in rivers that flowed on the Guianan and Brazilian
shields.
DNA analysis confirmed this hypothesis and showed that the piranha
populations present in the Amazon flood plain but situated 100 metres above
sea-level have been in existence for no more than 3 million years. Hitherto,
certain specialists had suggested that the present-day piranha species had
arisen in the lower sections of the great rivers of South America. The
researchers thought that from centres of speciation, piranhas would
subsequently have dispersed to colonize the more upstream reaches of the
river system. However, the results of the study give sustenance to another
scenario.
As per that new hypothesis, during the marine incursion phase some piranha
populations would have survived in the upstream parts of the network. Such
populations would have differentiated into speciesfollowing the fragmentation
of their zone of distribution, but probably also in response to ecological
constraints specific to the basin where they were kept in isolation from each
other. Once the ocean had regressed again, 3 million years ago, these
piranhas could finally have dispersed downstream, finding their way back to
the Amazons lowland plain which would have served as a gathering ground for
biodiversity. What now remains to be found are the ecological parameters that
could have favoured the diversification of piranha populations so confined to
the upper reaches of the river network.
One of the hypotheses advanced highlights water quality as a factor in
stimulating ecological and morphological differentiation of species. The
field survey observations indicated that some of the species were highly
localized, in both geographical and ecological terms. For example,
Serrasalmus hollandi is mostly found in turbid, sediment-laden waters flowing
down from Andean mountain streams. In contrast, a new species the biologists
discovered, lives in the same hydrographic basin but only in rivers with
crystal-clear waters bearing very little sediment content.
However, water quality cannot be considered as the sole factor behind
speciation, seeing that a third piranha species was found living in either of
these two categories of river. The research results as a whole suggest that
the superimposition of factors associated with geographical history and
ecological conditions, intervening at different spatial and temporal scales,
is responsible for the diversification of the piranhas. This is an
evolutionary progression which should be transposable to other fish
communities inhabiting South American waters.
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