Heterogeneity of Atlantic blue crab (Callinectes sapidus) invasion stages and population structure in the Adriatic Sea

Biological invasions are a well-documented driver of marine biodiversity change, yet quantitative frameworks to classify invasion stages and guide management remain limited. In the Adriatic Sea, the Atlantic blue crab (Callinectes sapidus) is an invasive alien species (IAS) which is widely distributed, reportedly altering ecosystem function and impacting local socio-ecological wellbeing. In response to these challenges, this study aimed to provide a tool which can assess population structure together with invasion dynamics in order to support management action. A coordinated monitoring programme was conducted through the Interreg Italy-Croatia 2021-2027 “BlueDiversity” project, during which IAS across eight pilot areas were monitored. Ecologically relevant morphometrics and demographics were recorded for individuals captured in the traps. Population structure was investigated in four Adriatic regions focused on coastal-marine areas, including estuaries and lagoons protected under Natura 2000. The four regions include: north, central east, central west and south. Morphometric traits (carapace width, length, thickness and weight), demographic indicators (juvenile-to-adult ratio, sex ratio), and catch-per-unit-effort were integrated into the Composite Invasion Index (CII scales from 0 to 10) to classify populations as Arrival, Establishment, or Expansion. Spatial variation in morphology and demographics revealed invasion-stage patterns consistent with ecological theory. Northern populations were adult-dominated and abundant, reflecting intermediate establishment. Central Eastern populations were dominated by adults and relatively less abundant. In contrast, Central West and southern populations exhibited high juvenile proportions, balanced to female dominated sex ratios, and elevated abundance, consistent with active expansion. These patterns aligned with environmental gradients: nutrient-rich northern systems supported stable populations, whereas warmer, more saline southern regions favored recruitment. These findings demonstrate an integrated morphometric and demographic index which provides a scalable framework to operationalize invasion-stage assessments.