Predation by Native Mediterranean Species on the Invasive Blue Crab: Experimental Evidence from the Common Octopus and the Loggerhead Sea Turtle

The rapid spread of the invasive blue crab (Callinectes sapidus) in Mediterranean coastal ecosystems poses ecological and socio-economic challenges, highlighting the need to explore natural control mechanisms. We conducted controlled aquarium experiments to assess the functional size preference and feeding rates of two native Mediterranean predators: the common octopus (Octopus vulgaris) and the loggerhead sea turtle (Caretta caretta). Two sets of trials were performed: (1) size preference experiments, in which individual predators were simultaneously offered crabs of varying sizes; and (2) ad libitum feeding experiments to estimate daily consumption rates. Octopuses consumed only the soft tissues biomass (STB, ~54% of total crab biomass), reaching maximum intake rates of 454 ± 71 gWW·d−1 in adult individuals. Sea turtles ingested whole crabs, including the exoskeleton, with total consumption rates up to 815 ± 592 gWW·d−1 of total biomass (TB) in adults. Subadult octopuses exhibited shorter handling times for small crabs, although neither predator showed a statistically significant size preference. Our findings indicate that both predators can consume substantial amounts of blue crab biomass, but O. vulgaris—due to its higher feeding efficiency and ecological abundance—might play a more prominent role as a natural biocontrol agent. These results provide valuable insights for integrated management strategies aimed at mitigating the ecological impact of C. sapidus in the Mediterranean.