Predation by Herring Gulls ( Larus argentatus Coues) on two rocky intertidal crab species [ Carcinus maenas (L.) & Cancer irroratus Say

Abstract

The vulnerability of crabs to predation by Herring Gulls ( Larus argentatus Coues) was investigated in mid-intertidal pools. Three tethering experiments tested 1. (1) the importance of spatial refuges (algal and crevice) for the prey population of Carcinus maenas (L.) 2. (2) if Carcinus maenas and Cancer irroratus Say have different mortality rates because Carcinus maenas has a more cryptic carapace on mussel substratum. Carcinus maenas in algal refuges had significantly higher survival than crabs in the exposed treatment. This effect was apparent only in crevices of ≥ 6 cm, where survival was 90%. Comparing the two crab species, mortality of Cancer irroratus was consistently high (≥ 83%) regardless of pool assignment; Carcinus maenas mortality was pool-specific. It ranged from 16 to 50% in the pool with mussel bed, but reached 83% in a pool predominated by coralline algae substratum. This suggests that 1. (1) Cancer irroratus is less cryptic in mid-intertidal habitats where dark backgrounds such as mussel substratum can predominate 2. (2) spatial refuges in the mid-intertidal will buffer the effects of predation, especially for the cryptically colored Carcinus maenas (refuge effectiveness with Cancer irroratus was not tested). We suggest that predation by Herring Gulls ( Larus argentatus Coues) in the mid-zone may cause Cancer irroratus to seek refuge in low intertidal pools and sublittoral areas where urchin barrens (lighter substratum) are more common, but inability to withstand high temperatures prevents Cancer irroratus from inhabiting high-intertidal pools.