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Crabs are invading Italian waters and threatening the local fishing economy 鈥� potentially jeopardising the global supply of clams in the process.

Atlantic blue crabs (Callinectes sapidus) are native to the eastern coasts of North and South America, where they are celebrated by diners as a summer delicacy. However, the predatory crabs have been spreading through the Mediterranean in recent decades and are feasting on young clams and mussels, along with other marine life.

These crabs 鈥渃an settle almost everywhere in the Mediterranean Sea鈥�, says at the University of Salento in Italy. Blue crabs have a wide diet and can live in varied water temperatures and salinities, which makes them 鈥渁 perfect invader鈥�, ecologically speaking, he says. The most damaging invasive species are usually well-adapted to a variety of conditions and can out-compete native wildlife for limited food and space. In the case of these blue crabs, they are also wiping out species that are harvested for human consumption.

Atlantic blue crabs were first recorded in European waters more than a century ago, but 鈥渦ntil last year, their impact was limited to a few areas鈥�, says of the Institute for Environmental Protection and Research in Italy. This year, blue crab numbers exploded, and the species spread to new coasts, estuaries and lagoons. 鈥淚t has [few] predators in our waters and is very competitive,鈥� Raicevich says. The crabs have the coasts of countries including Spain, Greece, Morocco, Portugal and Denmark.

Italian fishing communities are especially worried about their country鈥檚 clam supply, as the nation is the largest producer of clams in Europe and one of the largest in the world after China and South Korea. In some areas, blue crabs have eaten around half of the adult clam population, and young clams have 鈥渧irtually disappeared鈥�, says Raicevich. The loss of juvenile shellfish could undermine the food supply for years to come, as it takes two to three years for a clam to reach harvestable size.

The invasion is primarily taking place in lagoons in the north of the country, such as the Po river delta in the Veneto region, where locals have taken to catching and eating as many of the crabs as they can. But the Italian market for blue crab isn鈥檛 as well established as in the United States, and demand is limited. As such, harvesting the crabs doesn鈥檛 appear to be stopping their spread.

According to , around , including over 100 tonnes in August alone. The government declared a 鈥渟tate of calamity鈥� in July and announced a project to deploy 300 crab traps in the area.

Experts aren鈥檛 sure if or when the crabs鈥� spread will slow, but some are optimistic that the crabs鈥� population boom won鈥檛 be forever. Once the species runs out of food and space, 鈥渋t can鈥檛 last鈥�, says Mancinelli. Until then, he says trapping blue crabs and selling their meat and shells could buffer economic losses in the region.