A COMPARISON OF THE STRENGTHS OF GASTROPOD SHELLS WITH FORCES GENERATED BY POTENTIAL CRAB PREDATORS

Carcinus maenas, Liocarcinus puber and Cancer pagurus are thought to b e three likely crab predators of the gastropod Calliostoma zizyphinum. In order to compare the strengths of predators and their prey, the wh ole shell and aperture lip strength of white and pink Calliostoma morp hotypes and the maximum forces exerted by the chelipeds of three crab species were measured. Although white shells were thicker than pink sh ells, Calliostoma colour morphotypes did not differ significantly in e ither the force required to break the shell lip or the whole shell. Bo th Liocarcinus puber and Carcinus maenas have dimorphic chelipeds and their 'crusher' chelipeds deliver almost double the forces generated b y the 'cutter' chelipeds. In contrast, Cancer pagurus has monomorphic chelipeds both delivering similar forces. When compared with Calliosto ma shell strength, the forces generated by the `crusher' chelipeds of most L. puber tested were, in general, sufficient to break the shell l ip of Calliostoma shells, whereas forces generated by the 'cutter' che lipeds of only the larger individuals were sufficient to break the she ll lip. In C. maenas, forces generated by both the 'cutter' and 'crush er' chelipeds often exceeded the minimum recorded force required to br eak the shell lip and the 'crusher' cheliped reached the minimum force required to break whole Calliostoma shells. Both chelipeds of all C. pagurus tested generated forces in excess of the minimum required to b reak the shell lip, and the proportion of individuals capable of gener ating the minimum force required to break the whole shell increased wi th the size of the crab. Carcinus maenas and Cancer pagurus us were ca pable of breaking both the shell lips and the whole shells of a wider range of shell sizes than L. puber.