Cracking the cuisine: Invasive European shore crabs (Carcinus maenas) select a menu of soft-shelled mussels over cockles

The European shore crab, Carcinus maenas, is known to feed extensively on a variety of benthic species, and can affect commercially important bivalve fisheries. As a marine invasive species, predation by C. maenas can have ecological and commercial effects in its non-native range. We investigated potential predatory effects of non-indigenous C. maenas on native bivalves, and if shell strength of bivalve prey influenced predation susceptibility. The main prey species tested were ecosystem-engineering mussels (Mytilidae, Xenostrobus inconstans), and commercially valuable mud cockles (Veneridae, Katelysia peronii.). In separate field experiments, crabs were offered either a choice between both bivalve species or non-choice for each species separately. Male and female crabs equally consumed mussels across both choice and non-choice experiments. The proportion of cockles consumed was significantly lower than that of mussels in choice experiments, and no cockles were consumed in the non-choice experiment. Bivalve compressive shell strengths were tested separately with a universal testing machine. The breaking force required to crush shells of adult K. peronii was much higher than the claw strength of adult crabs, but the significantly lower shell-breaking forces required for X. inconstans make the mussels vulnerable prey. Additional compressive shell strength tests on other native bivalve species and juvenile Katelysia showed that they were susceptible to crab predation. Results indicate that predation by non-indigenous C. maenas on soft-shelled mussels and juvenile cockles could impact native bivalve abundances. Shell strength measurements of potential prey are a suitable approach to assess the risk for ecological and economic impacts of this globally invasive crab.