Why Are Blue Crabs Invading Europe? Italy's Blue Crab Problem

How Atlantic blue crabs reached Europe

The blue crab, Callinectes sapidus, is native to the western Atlantic, from Nova Scotia to Argentina. Its first known European record dates from about 1900 on the Atlantic coast of France. Over the following decades scattered crabs turned up in the Netherlands, Germany, Denmark and the North Sea, but they stayed rare in these cold waters.

The Mediterranean was different. Blue crabs were recorded in the Aegean Sea and off Egypt by the 1940s, and in the northern Adriatic near Grado and Venice by around 1950. By the 1950s and 1960s they were established in the eastern Mediterranean, especially around Greece and Turkey.

How did they cross the ocean? The most likely route is ballast water: ships take on seawater in one port to keep them stable and release it in another. Blue crab larvae drift in the plankton for a month or more, long enough to survive a crossing. Genetic studies show that Mediterranean blue crabs have very little genetic variety, which suggests the whole population descends from only a few introductions.

Why blue crabs thrive in the Mediterranean

Several features make the blue crab an unusually successful invader:

• Perfect habitat. The Mediterranean is lined with brackish lagoons, river deltas and estuaries, exactly the habitats blue crabs use in Chesapeake Bay.
• Warm water. Crabs grow, molt and breed faster in warm water. As the Mediterranean has warmed, conditions have become even better, especially in the cooler western basin.
• Huge numbers of eggs. A female can release from several hundred thousand to a few million eggs in each batch, and she may spawn more than once.
• A generalist diet. Blue crabs eat clams, mussels, oysters, snails, worms, fish, other crabs, dead animals and plants.
• Toughness. They tolerate water from almost fresh to fully salty and survive low oxygen levels that kill many native animals.
• Few specialist enemies. Native fish, octopuses and birds do eat blue crabs, but not enough to control them.

In 2006, a review of alien species in the Mediterranean placed the blue crab among the region’s 100 worst invasive species.

The 2023 outbreak in Italy

For decades, blue crabs were only an occasional sight in Italy. From the mid-2010s their numbers rose quickly in the Gulf of Trieste and the northern Adriatic, and they spread to the Ionian and Tyrrhenian seas, Sardinia and Sicily.

The crisis peaked in the summer of 2023 in the lagoons of the Po Delta, such as the Sacca di Goro and Sacca degli Scardovari. These lagoons are among Europe’s most important areas for farming clams, especially the Manila clam. Blue crabs swarmed into the farms and ate huge quantities of young clams, as well as mussels and oysters. Fishers also reported nets cut by the crabs’ claws and fish damaged in their nets.

The story made headlines across Europe. Industry groups estimated losses to Italian shellfish farming at up to about 100 million euros. In August 2023 the Italian government set aside 2.9 million euros to help fishers catch and dispose of the crabs, and it later appointed a special commissioner to coordinate the national response.

Where blue crabs have spread in Europe

By the 2020s, blue crabs were established around almost the entire Mediterranean coast.

In the Ebro Delta in Spain, studies have described the blue crab becoming one of the dominant animals in some lagoons within a few years of arriving. In the eastern Mediterranean, a second invasive blue swimming crab, Portunus segnis, arrived from the Red Sea through the Suez Canal.

How Europe is responding

Because eradication is considered impossible, most countries are trying to reduce the damage. Fishers are paid or encouraged to trap blue crabs in large numbers in key lagoons, and some clam farms have tried protective nets and barriers to keep crabs out.

Another approach is to turn the pest into a product. The blue crab is a valuable seafood in the United States, and Greece and Turkey have fished it for decades. Since 2023, Italian chefs, markets and fishers have promoted blue crab dishes, and exports have grown. Tunisia, where an invasion of the related Portunus segnis devastated fisheries, built a successful export fishery for blue crabs, which is often cited as a model.

Scientists warn that commercial fishing alone may not keep numbers low, and that pressure to profit from the crabs could discourage efforts to reduce them. They also point out that some invasions settle down over time as predators and diseases catch up, though nobody knows whether this will happen with the blue crab.

Other invaders of European seas

The blue crab is one of hundreds of alien marine species in the Mediterranean. Many, like the lionfish, arrived from the Red Sea through the Suez Canal, while others came in ballast water or on ship hulls. The lionfish story, explained in why are lionfish invasive?, shows how a predator with few enemies can spread rapidly in a warming sea. Native species such as the loggerhead turtles described in are there Mediterranean sea turtles? now share their coasts with these newcomers. For Italy’s native wildlife, see wild animals of Italy.