Blue and green: A portrait of two portunoidea crabs from away and their interactive effects on catch

Biological invasions and climate-driven range expansions are generating novel assemblages of interacting species with ecological and socio-economic consequences. While competition between invasive and native species has been widely documented, interactions between invasive and range-expanding species remain comparatively underexplored, despite their growing relevance. This study set out to compare a "reciprocal invasion" of two Portunoidea crabs-American blue crab (Callinectes sapidus) and the European green crab complex (Carcinus spp.)-between the Mediterranean Sea (Italy), where the green crab is native and the blue crab a recent invader, and the Gulf of Maine (USA), where the green crab is an established invader and the blue crab is expanding its range. Through a series of trapping surveys, we found a consistent negative relationship in catches between these two species across regions, suggesting that C. sapidus is a competitive dominant regardless of geographic context. Further, we observed different responses to temperature and salinity between species, suggesting that their coexistence is likely limited by both competitive exclusion and environmental factors. Across both regions, blue crab catches peaked during warm seasons and higher temperature-salinity conditions, whereas green crabs dominated cooler periods and lower salinities, revealing consistent seasonal and environmental niche segregation that reinforces competitive exclusion patterns. This study highlights the importance of considering both environmental niches and biotic interactions as they jointly shape invasion outcomes, and demonstrates the utility of cross-region comparisons for better understanding the conservation implications of novel species interactions in an ever-changing ecosystem.