Multiscale modelling of dispersal pathways for the invasive blue crab larvae (Callinectes sapidus) in the Mediterranean Sea

The blue crab ( Callinectes sapidus ) is an invasive species present in the Mediterranean Sea since the mid-twentieth century, which has undergone a significant population increase in the last decade. This demographic explosion raises urgent concerns, necessitating detailed information on its dispersal dynamics at multiple scales. To investigate the dispersal pathways and connectivity patterns of C. sapidus , we conducted a biophysical modelling study over an eleven-year period (2010–2020), combining a Lagrangian particle-tracking tool with two hydrodynamic models of contrasting resolutions: a basin-scale model (MedMFC, ~ 4 km) and a high-resolution regional model (MARS3D, 1.2 km). By analyzing the path-use density of millions of simulated larvae, we identified major links connecting distant sub-basins, while highlighting the critical role of high-resolution modelling in resolving coastal retention features. An ensemble analysis of larval fate revealed strong spatial heterogeneity: while distant settlement drives recruitment in sink areas such as the Ligurian Sea and Eastern Corsica, local retention is the dominant mechanism in semi-enclosed environments such as the Gulf of Lion, the Gulf of Gabès and the Adriatic Sea, particularly during the summer spawning season. Connectivity matrices identified robust regional clusters, notably linking Tunisia, Sicily, Sardinia, and the Tyrrhenian coast, suggesting that islands act as critical stepping stones in the colonization process. These findings provide a quantitative baseline of larval exchange networks, highlighting key areas for future investigation to support the development of operational tools for management strategies.