PREDATION BY HERRING-GULLS (LARUS-ARGENTATUS COUES) ON 2 ROCKY INTERTIDAL CRAB SPECIES [CARCINUS-MAENAS (L) AND CANCER-IRRORATUS SAY]

The vulnerability of crabs to predation by Herring Gulls (Larus argent atus Coues) was investigated in mid-intertidal pools. Three tethering experiments tested (1) the importance of spatial refuges (algal and cr evice) for the prey population of Carcinus maenas (L.) and (2) if Carc inus maenas and Cancer irroratus Say have different mortality rates be cause Carcinus maenas has a more cryptic carapace on mussel substratum . Carcinus maenas in algal refuges had significantly higher survival t han crabs in the exposed treatment. This effect was apparent only in c revices of greater-than-or-equal-to 6 cm, where survival was 90%. Comp aring the two crab species, mortality of Cancer irroratus was consiste ntly high (greater-than-or-equal-to 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) Cancer irroratus is less cryptic in mid-intertidal habitats where dark backgrounds suc h as mussel substratum can predominate and (2) spatial refuges in the mid-intertidal will buffer the effects of predation, especially for th e cryptically colored Carcinus maenas (refuge effectiveness with Cance r irroratus was not tested). We suggest that predation by Herring Gull s (Larus argentatus Coues) in the mid-zone may cause Cancer irroratus to seek refuge in low intertidal pools and sublittoral areas where urc hin barrens (lighter substratum) are more common, but inability to wit hstand high temperatures prevents Cancer irroratus from inhabiting hig h-intertidal pools.