A fisherman holds a blue crab in the lagoon of Scardovari, south of Venice, Italy, on August 11, 2023. The blue crab, native to the North American Atlantic coast, has been spreading across the Mediterranean for years and is now causing a crisis in some of Italy's coastal areas. Credit: Piero Cruciatti/AFP via Getty Images.
The northern Adriatic Sea has been invaded by Callinectes Sapidus, a blue crab species native to the east coast of the United States. It is an adaptable species that reproduces very quickly, and it is threatening the clam farming industry of the Po River delta in the Veneto and Emilia-Romagna regions, which normally produces about 15,000 tonnes of clams per year.
The Italian government has allocated nearly €3 million for the capture and disposal of the species, authorizing fishermen to catch as many crabs as possible. But it is unlikely that this will eradicate the crab.
Nature Italy spoke to experts from Italy and Tunisia, which previously faced a blue crab invasion, to understand the impacts.
When and how did the new species arrive?
Callinectes Sapidus, a crustacean of the decapods order that also includes lobsters, crayfish, shrimps and prawns, was first observed in the Mediterranean Sea in 1949, where it was probably transported by the ballast waters of transoceanic ships.
It started to grow in the northern part of the Mediterranean, occupying especially lagoon areas due to their salinity and food availability. “The colonization took some time, it is a slow process,” says Gianluca Sarà, marine ecologist at the University of Palermo. Before invading the Po River Delta, Atlantic blue crabs have been spotted in other locations in Italy. Established populations were first detected in 2014 in the lagoon of Lesina and Varano, in Apulia.
Another blue crab species, native to the Red Sea and the Persian Gulf and called Portunus Segnis, was first detected in Egypt at the end of the 19th century, where it probably arrived through the Suez Canal. In 2015 it invaded the Gulf of Gabes, in southern Tunisia. “Fishermen unloaded hundreds of tons of blue crabs”, says Jamila Ben Souissi, an expert in Mediterranean biodiversity at the University of Tunis. “In their nets there was nearly nothing left apart from blue crabs and damaged fish that could not be sold,” she recalls.
Until recently the two crab species have been living separately. “Callinectes Sapidus is more tolerant to cold waters, whereas Portunus Segnis is accustomed to warmer,” Sarà says.Now however, according to Ben Souissi, they are starting to coexist in sizeable numbers in the Kerkennah Islands, in the Gulf of Gabes. “We are starting studies to understand whether one species will drive the other away.”
Is the blue crab invasion linked to climate change?
Sarà’s team has measured the oxygen consumption rate of Callinectes Sapidus at different temperatures and found that even if it can withstand temperatures as high as 40°C, its metabolism is optimal at around 24°C. “Mediterranean waters are warming up due to climate change and the number of days and places where the temperature nears the optimal one is growing and shifting northward”, says Sarà who believes that this has contributed to the displacement of the crab towards the northern Adriatic.
“But there are also local factors at play”, he comments. “The drought affecting the Po River in 2022 led to an expansion of saltwater [in the Delta], which could have favoured the movement of females towards the sea where eggs rupture more easily.”
Ben Souissi believes that climate change is an aggravating factor, but not the most important one, noting that the blue crab invasion occurred in Tunisia, but not in Libya. “The ecosystem degradation of Tunisian seas caused by pollution, overfishing and illegal fishing favoured the species, whereas in Libya the marine ecosystem is healthier”.
Should we kill blue crabs or learn to live with them?
A team led by Lucrezia Cilenti, at the Institute for Marine Biological Resources and Biotechnology of the National Research Council, has partnered with regional authorities and fishers associations to develop a management strategy. “To control the expansion, one should fish preferentially females, that can be found mostly in the canals that connect the lagoon to the sea, since they prefer more saline waters than males,” she says.
For the northern Adriatic, Sarà suggests the use of predictive models to identify vulnerable sites, and collect data for the whole year to avoid another invasion next summer. “We knew blue crabs were growing in number and we did nothing,” he says. “Let’s not repeat the same mistake again”.
However, promoting the consumption of blue crabs could forge a new industry. In 2015, the Tunisian government allocated funds to boost Red Sea crab exports, and today there are nearly 50 processing plants serving international markets. “The largest plant last year exported 5,200 tonnes of blue crab, and employs thousands of people,” Ben Souissi says. “At first fishers wanted this species to disappear, but now they are asking the authorities for regulations to protect it.”
What will be the long-term impacts on ecosystems?
Aside from the immediate damage to fishing and clam farming, the impact of the blue crab invasion in the Mediterranean is hard to measure. Sarà and his team are about to publish two studies on the effects of crabs on sediment and on biodiversity loss. “They could increase water turbidity, decreasing the light that filters on to the seabed, thus impacting photosynthetic species such as algae and seagrass”, explains Sarà, who recommends more studies about the medium and long-term possibilities.
