Size related changes in vulnerability to predators and spatial refuge use by juvenile Iceland scallops Chlamys islandica

We examined size-related changes in vulnerability of juvenile Iceland scallops Chlamys islandica to crab and asteroid predators using field experiments in the Mingan Islands, northern Gulf of St. Lawrence, eastern Canada. Vulnerability to both crabs and asteroids markedly decreased with scallop size, Likely because of increases in the effectiveness of the scallop's antipredator defenses. Multiple-choice experiments showed that the crabs Hyas araneus and Cancer irroratus preyed mainly upon small scallops (10 to 30 mm). At our study site H. araneus was the most abundant decapod and was likely the major predator of scallops. Small Iceland scallops were generally found in crevices, byssally attached to the undersides of dead shells and rocks. Use of these refuges did not show diel periodicity but decreased markedly with scallop size, attaining 0% for >60 mm scallops. Tethering experiments indicated that refuge use decreased vulnerability to both crabs and asteroids. However, the advantage of refuge use, in terms of reduced predation risk, varied with scallop size, being maximal for 15 to 30 mm scallops and only marginal for larger, less vulnerable scallops.