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Guest blog post by Chiara Facca

The Atlantic blue crab (Callinectes sapidus) is ranked among the 100 worst invasive species in the Mediterranean Sea. In the spring and summer of 2023, the Northern Adriatic experienced an unprecedented demographic outbreak of the species. To understand why this explosion happened so quickly and how to manage it, a team of researchers from Ca’ Foscari University of Venice and other Italian institutes set out to study the crab’s biology and ecology. The results have been published in the open-access journal Aquatic Invasions.

From Precaution to Emergency

Atlantic blue crab (Callinectes sapidus). Images credit to Marco Boschiero.

The continuous spread of the species in the years leading up to our study prompted us to monitor its population, as very little was known about it in our area.

explains Chiara Facca of Ca’ Foscari University of Venice and one of the researchers.

Because the blue crab is a highly versatile predator that targets bivalve mollusks, researchers were deeply concerned about the potential threats to local sectors, particularly traditional small-scale lagoon fisheries and Manila clam farming in the region.

The original goal was to prevent future damage, but right at the start of the study, the crab’s population turned into a real emergency, forcing the team to adapt their research objectives to manage the crisis.

The “Small Scale” Advantage: A Perfect Storm for Proliferation

One of the most critical discoveries from the research is that these invasive crabs are completing their entire, complex life cycle within an incredibly small area comprising of a spatial gradient of just 2 to 9 kilometers separating riverine, lagoon, and marine habitats.

Why does this matter? In the crab’s native range, such as the Chesapeake Bay in the United States, female blue crabs often have to migrate massive distances, up to 150-200 kilometers, to reach the higher-salinity open sea to spawn. This long migration often forces females to overwinter mid-journey as temperatures drop, significantly extending the time it takes to successfully spawn.

In the Northern Adriatic, the close proximity of the Brenta River, the Venice Lagoon, and the Adriatic Sea allows the reproductive cycle to occur much more rapidly. Because females do not have to undertake long migrations, they avoid the need to overwinter, which likely allows them to spawn eggs two or three times within a single reproductive season. This spatial compression, combined with the crabs’ high fecundity – averaging over 2 million eggs per mass, with peaks reaching nearly 6 million – accelerates the species’ proliferation and makes the population incredibly difficult to control.

A Surprising Discovery at the River Mouth

We were remarkably surprised by the speed at which the species adapted and proliferated across the region. Our findings demonstrate that the Northern Adriatic Sea provides the ideal environmental conditions for this species to thrive.

the team explained.

However, the data revealed a fascinating paradox regarding where the crabs were found versus how healthy they were.

While male crabs were highly abundant at the Brenta River mouth station (an oligohaline, or low salinity, environment), they exhibited a significantly poorer “relative condition factor” (a measure of weight and health) compared to crabs in the lagoon and marine stations. The study suggests this lower condition factor is due to the high energy the crabs must consume for osmoregulation in lower-salinity waters, coupled with a lack of their favorite food source, the bivalve mollusks, which are far more abundant in the nearby coastal and lagoon habitats.

Targeted Management: What Can Be Done?

Eradicating the Atlantic blue crab is virtually impossible due to its high adaptability, rapid mobility, and ubiquitous distribution across the Mediterranean. However, the researchers emphasize that targeted management strategies are highly feasible.

Control measures should exploit specific life-history events observed in the study. For instance, the research confirmed that females consistently migrate to higher-salinity marine waters during the spring and summer to spawn. By concentrating harvesting efforts on females during this spawning period at critical geographic bottlenecks, such as lagoon inlets and river mouths, authorities could significantly control population abundance. This targeted approach is essential for mitigating the severe economic impacts on local ecosystems, traditional small-scale fisheries, and clam aquaculture.

Looking Ahead

While this study serves as a critical baseline for understanding the blue crab dynamics in the Northern Adriatic Sea, the research team warns against making assumptions. Many facets of the species’ biology, ecology, and behavior in this invaded range remain unexplored, and these traits may not perfectly mirror those observed in its native habitat or other invaded regions globally. There is still a vast amount of research to be done to fully comprehend and manage this unprecedented ecological phenomenon.

Original source:

Boschiero M, Facca C, Cavraro F, Redolfi Bristol S, Gavioli A, Riccato F, Zucchetta M, Franzoi P (2026) Ecology and biology of Callinectes sapidus in the Northern Adriatic Sea: could the small spatial scale explain its outbreak? Aquatic Invasions 21(1): 49-72. https://doi.org/10.3391/ai.2026.21.1.180751