Invasive blue vs. local Green: analysis of substrate preference of two crab species, Callinectes sapidus and Carcinus aestuarii

Introduced primarily via ballast waters, the Atlantic Blue Crab, Callinectes sapidus, has spread throughout the Mediterranean Sea exhibiting a highly invasive behaviour. In introduced ecosystems it is posing a threat to local species and economic activities. This study compares, for the first time, habitat preference of C. sapidus and the native green crab, the Mediterranean Shore Crab Carcinus aestuarii, in the Venice Lagoon. Sediment samples from natural sandy bottoms, as well as from natural and artificial saltmarshes were used to assess substrate preference (time spent on each substrate) of both species under controlled laboratory conditions. Sediment composition was characterized by granulometric analysis, to assess differences in the sediment structure of the samples. Behavioural observations indicated that C. sapidus exhibits a preference for substrates with larger grain size, that characterized both natural bottoms and artificial saltmarshes, whereas C. aestuarii showed no specific substrate preference. Moreover, C. sapidus demonstrated significantly higher burrowing activity, likely due to its adaptation to sandy environments and burrowing behaviour as a predation avoidance strategy. Hydro-morphological alterations driven by both human-driven erosion and by restoration actions are increasing the sandy component of the Venice lagoon sediments across habitats. As a result, the blue crab may potentially thrive in wider areas. Our findings have implications for conservation and management, as the expansion of C. sapidus could further threaten C. aestuarii populations through competition and predation, exacerbating the decline of local artisanal fisheries.