Use of the intertidal zone by mobile predators: influence of wave exposure, tidal phase and elevation on abundance and diet

Linkages between predators and their prey across the subtidal-intertidal boundary remain relatively unexplored. The influence of tidal phase, tidal height and wave exposure on the abundance, population structure and stomach contents of mobile predatory crabs was examined on rocky shores in southwest Britain. Crabs were sampled both during the day and at night using traps deployed at high tide and by direct observation during low tide. Carcinus maenas (L.), Necora puber (L.) and Cancer pagurus (L.) were the most abundant species, being mainly active during nocturnal high tides. C. maenas was the only species that was active during nocturnal low tides, when it was observed mainly on the lower shore feeding on limpets. Individuals of all 3 species sampled during high tide were considerably larger than those sampled during low tide. Thus, sampling crab populations at low tide is likely to underestimate abundance and the extent of predation by crabs on rockyshore assemblages. During immersion, the relative abundance of each species was influenced by exposure to wave action and tidal elevation. All species were more abundant on the lower shore; C. maenas and N. puber were more abundant in sheltered locations, while C. pagurus was more abundant in exposed locations. Analyses of stomach contents from individuals captured at high tide revealed that chitons and limpets were the most common hard-shell prey taxa in the diet of these predators. The relative abundance of prey in gut contents was, however, not correlated with patterns of prey abundance. Our study indicates the importance of crabs as key intertidal predators and illustrates the strong trophic linkages between the subtidal and intertidal zones, which is likely to be a key factor influencing community structure on European shores.