By Joseph Milton
The elusive great white sharks of the Mediterranean Sea may be descended from a single small Australian population that lost its bearings while visiting South Africa 450,000 years ago.
The great whites (Carcharodon carcharias) were probably returning to the Antipodes but became trapped after passing through the Straits of Gibraltar, according to a team led by marine biologist Leslie Noble of the University of Aberdeen, UK.
The sharks have since made the Mediterranean their home because they reproduced there and, like salmon, the young always return to their birthplace.
Little is known about Mediterranean great whites--sightings are rare and tissue samples even rarer--but Noble and his colleagues teamed up with Turkish researchers to get access to samples from four sharks caught in fishing nets: two from Turkey, one from Tunisia and another from Sicily.
Their research, published November 17 in Proceedings of the Royal Society B suggests that a combination of climate change, high sea levels and strong ocean currents around the South African coast could have driven the migrating Australian sharks off-course, up the west coast of Africa and east into the Mediterranean. But because the initial population was small, genetic variability in modern Mediterranean sharks seems to be limited--indicating that a lack of diversity could threaten their future survival. Female sharks from the nearby Atlantic do not seem to be migrating to the region, where they could help to replenish the stagnant gene pool.
Lost at sea
The researchers sequenced an area of the four sharks' mitochondrial DNA--DNA that is passed onto offspring from the mother and encodes proteins from cells' energy factories. The team was then able to compare the genetic code with a bank of sequences derived from great whites in waters of different parts of the world, including South Africa, Australia and the Atlantic.
Noble says the team was surprised to find that the section of DNA sequenced was identical in three of the four Mediterranean sharks, and showed that they were most closely related to Australian great whites. The team had expected to see more affinity with the nearer Atlantic or western Indian Ocean populations.
Great white sharks were once thought of as a coastal species, but research has shown that they migrate long distances in the open ocean--although scientists do not know exactly why. Tagged sharks have been seen traveling between the coasts of South Africa and southern Australia, and the authors suggest that it was probably during one of these excursions that a group took a wrong turn.
The researchers used a molecular dating technique based on the number of differences between the DNA of the Mediterranean and Australian sharks to estimate that the sharks got lost during the Pleistocene epoch, around 450,000 years ago. Noble says that this was an period between ice ages: a time of high sea levels, climate change and, perhaps most importantly, an unusually fast-flowing ocean eddy off the east coast of South Africa called an Agulhas ring--which may explain why the sharks went so far astray.
The warm Agulhas Current flows down the east coast of Africa, but periodically an Agulhas ring carries its waters around the southern tip of the continent and into the Benguela Current off the west coast. "When sharks follow the Agulhas Current, the cooler waters of the Benguela probably alert them to turn east," says Nelson, "but an Agulhas ring is like a warm-water bubble." A group of sharks swimming in one of these bubbles could miss the turning and find the western coast of Africa between it and its desired destination.
The researchers speculate that the sharks then swam north until the Mediterranean basin gave them a chance to head east again. Once in the basin, they may have become trapped by the peninsulas and narrow channels of the Mediterranean.
A population in peril
Paulo Prodöhl, an evolutionary geneticist at Queen's University Belfast, UK, says that although the finding "comprises a precious and unique data set, the sample sizes are really too small to draw conclusive inferences." But he admits that because shark samples are so hard to get hold of, "you have to work with what you can get."
"We recognize the sample-size problem," says Noble. "We're trying to get another 50 Mediterranean samples, which could dramatically change our inferences--we're very keen to access museum material."
But, he says, "I don't think it removes the central tenet--that as far as we're aware, a significant proportion of the Mediterranean sharks are Australian in origin."
Noble also hopes that the work will highlight the plight of a potentially fragile shark population surviving in a polluted and over-fished sea. He says that great whites occupy a "pivotal role" in the Mediterranean, and the removal of top predators from other marine ecosystems has been disastrous.
"On the east coast of America, shark eradication has caused an 'ecological cascade'," says Noble. Populations of species on which sharks prey, such as seals and dolphins, have exploded, unbalancing the whole system. "It's been instrumental in helping kill off some of the shellfish," says Noble. "I wouldn't like to speculate on the consequences for the Mediterranean if this population became extinct."