Blue crabs: Alien stowaways in the Mediterranean

Issued on: 18/06/2021

By: Mairead DUNDAS

Pierre LEMARINIER|Antonia KERRIGAN|Julia GUGGENHEIM

Valérie DEKIMPE|Marie-Claire IDE

A captivating crustacean and a ferocious predator: in this edition we discover a transatlantic invader that's taking no prisoners in its quest for domination. Safe from its natural predators on the Atlantic coast of North America, the blue crab devours everything in its path, dealing a painful blow to an already fragile ecosystem.

Hitching a ride on boats

The blue crab is just one of around 900 alien species that have set up home in the Mediterranean Sea. They arrive here from foreign lands, often hitching a ride on boats. When shipping vessels leave a port they suck up water to help stabilise their load, known as ballast water, but also take with them a host of microorganisms, algae and fish eggs. This water is then released, along with its contents, when the boat reaches its destination. It's believed that the blue crab found its way to Europe in this way.

Biologist Pascal Romans points out that in an already polluted and overexploited sea, newcomers like the blue crab often prosper and flourish at the expense of the locals.

Fishermen to the rescue

The blue crab hasn't won yet, though, and fisherman Christophe Guinot is determined to preserve the biodiversity that has been key to his family’s livelihood for generations. While the species can never be eliminated from the Mediterranean, it can be controlled.

His answer? Fishing. Instead of the sustainable fishing encouraged for other species, the crab must be systematically removed and prevented from reproducing.

Global effort to control bio invasions

By the year 2024, the vast majority of boats in the world will be required to install systems that avoid the transfer of new species via ballast water.

Xavier Deval from BIO SEA oversees one technique that involves filtering the water and then passing it through UV lighting to neutralise the microorganisms. It's hoped that such measures will significantly reduce the quantity of this unwanted and potentially dangerous cargo.