Highly mobile predators are known to have an important influence on shallow-water habitats. There is, however, little information about the role of crabs on the ecology of rocky shores. Here I examine the extent and consequences of predation by crabs on shores in the North-East Atlantic. The specific aims of this thesis are to: investigate spatial variation in use of the intertidal by crabs along a horizontal gradient of exposure to wave action and a vertical gradient of tidal elevation; assess their use of the intertidal as a feeding area by examining stomach content composition; examine the extent of connectivity between the subtidal and intertidal habitats as a consequence of predation by crabs, investigate the ecological role of crabs as predators in the intertidal, and to assess the importance of behavioral and morphological factors in determining the outcomes of these predator-prey interactions. Field sampling showed that on shores in southwest Britain, the abundance of Carcinus maenas, Necora puber and Cancer pagurus varied between high and low-tide, with the vertical gradient of tidal height and horizontal gradient of exposure to wave action. Crabs were typically more abundant on the lower shore than on the upper shore. C. maenas and N. puber were more abundant on sheltered shores than on exposed shores, while C. pagurus showed the opposite pattern. Individuals captured at high-tide were on average larger than those captured at low-tide. Stomach content analysis of individuals captured with traps at high-tide showed that all three crabs feed on intertidal prey including limpets, chitons and algae. In a mark-recapture field experiment, I demonstrate the migration of sublittoral crabs into the intertidal during high-tide. Subsequent manipulative field experiments showed that predation by crabs can have a considerable effect on abundance of limpets. Laboratory experiments showed that Necora puber has a preference for smaller limpets, indicating that the population structure of these grazers may also be modulated by predation. On shores of differing exposure in Portugal I examine cheliped morphological variation of Eriphia verrucosa in response to variation in prey abundance. Chelipeds were typically larger on exposed shores, where hard shelled prey such as mussels were more abundant than they were on sheltered shores, which were dominated by chitons and algae. Predation by crabs therefore appears to have an important ecological role in shallow-water habitats by influencing intertidal prey populations and establishing an important trophic link between intertidal and subtidal habitats. The implication of predation by crabs on the ecology of rocky shores is discussed.

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