In the 18th century, Europe started sending boatloads of white settlers to Australia. But unbeknownst to these colonists, Australia had sent its own white contingent to set up colonies in Europe, around 450,000 years earlier. These migrants were sharks – great white sharks.

When Chrysoula Gubili from the University of Aberdeen compared the DNA of white sharks from around the world, she found a big surprise. The great white is the most genetically diverse shark studied so far but the Mediterranean fish are only distantly related to nearby populations in the North-West Atlantic, or even in South Africa. Their closest kin actually live half a world away in the Indo-Pacific waters of Australia and New Zealand.

Great whites have been recorded throughout the Mediterranean, mostly around the coast of Italy. After vague reports during the 19^th century, the first proper sighting came when a female was caught off the coast of Sicily in 1901. In a gruesome twist, her stomach contained three human corpses. Other encounters have been less grisly, but fishermen have found pregnant females and newborns. This means that the sharks aren’t just passing visitors; they’re using the Mediterranean as a breeding ground.

Gubili thinks that the European population was set up by a single founding female who got lost. Female great whites undergo long migrations of thousands of kilometres, but they tend to return to the place where they were born. However, it’s possible that some individuals lose the bearings on these monster treks. These navigational problems rarely amount to anything. But if the wayward female is pregnant, she might end up setting up an entirely new splinter group in a far-off corner of the world.

How did this wandering shark end up in the Mediterranean in the first place? Many species of fish have moved there from Australian waters via the man-made Suez canal, which cuts a path directly into European waters from the Red Sea. These are known as Lessepsian migrations, but they can’t explain the existence of white sharks in the Med, which happened much earlier in time.

Gubili’s samples suggest that this population was set up around 450,000 years ago, towards the end of the Pleistocene period. There was no canal around back then, so the founding shark must have swum the long way round: across the Indian Ocean, around the tip of Africa and through the Atlantic.

The current that flows through this route – the so-called ‘Agulhas leakage – was in full force during the late Pleistocene period, when the Mediterranean great whites arrived in their new home. At the same time, a major global freeze had sent sea levels falling. That exposed large tracts of underwater land around South-East Asia and Australia, shutting off the route between the Indian and Pacific Oceans. With the way east blocked, migrating fish such as great whites turned west.

Strong currents would have led them north along the African coast. Once north, Gubili says that an innate tendency to swim east to their birth waters would have trapped them in the Mediterranean. The sharks that swim in the Med today are the descendants of this accidental voyage.

This singular ancestry is potentially bad news. Shark populations in the Mediterranean are plummeting fast. If the great whites were the result of a one-off accident, it means that their dwindling numbers are unlikely to be replaced by new migrants from elsewhere in the world. It’s possible that new sharks could swim in from the eastern North Atlantic but the great white is very rare there, and radio tags suggest that the Atlantic individuals migrate along the US coast rather than east into Europe. It seems that this group of great whites is closed, isolated, and shrinking.

After hearing tales of mangled Italian swimmers, one might wonder if this is a bad thing. The answer is invariably yes. To put things in perspective, shark attacks are vanishingly rare in Europe and Italy – where most Mediterranean great whites are found – has only experienced 10 since 1847.

On the other hand, the great white shark is at the very top of the food chain and as such, the balance of an entire web of species rests in its jaws.  The message from other parts of the world is clear – when sharks are killed off, other species suffer and entire communities can collapse (often to the detriment of local humans).

Reference: Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2010.1856

Image: by Terry Goss

More on sharks:

• Prehistoric great white shark had strongest bite in history
• Shark-hunting harms animals at bottom of the food chain
• Male and female mako sharks separated by invisible line in the sea
• Pocket Science – a nursery for giant sharks
• Pocket Science –the mystery of the shark-bitten fossil poo
• Widely set eyes give hammerhead sharks exceptional binocular vision

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November 16th, 2010 Tags: Australia, great white shark, Mediterranean, migration
by Ed Yong in Animal behaviour, Animal movement, Animals, Evolution, Fish, Genetics, Sharks | 4 comments | RSS feed | Trackback >