Sources of predatory mortality in the bay scallop Argopecten irradians (Lamarck): interactions with seagrass and epibiotic coverage

Abstract

Field observations and experiments were conducted to identify the sources of predatory mortality in adult bay scallops Argopecten irradians concentricus Say and to determine the physical factors that affected the rate of predation. Ring-billed gulls Larus delawarensis Ord, herring gulls L. argentatus Coues, and knobbed whelks Busycon carica Gmelin were responsible for the mortality of scallops placed in the field. Predation intensity by these predators varied significantly with depth of water coverage and habitat type. Predation by gulls was more intense on intertidal scallops than on scallops that were barely subtidal (covered by <5 cm of water), and less intense on scallops within shoalgrass Halodule wrightii Acherson beds than on scallops on bare sand bottom. Predation by whelks was less intense on scallops within subtidal eelgrass Zostera marina L. beds than on unvegetated subtidal sand in April, but a similar pattern in August was not significant. Epibiotic coverage by macroalgae and invertebrates did not alter the rate of predation in any of the experiments. The results from these experiments suggest that seagrass acts as a partial refuge from predation for bay scallops, and in part explains the marked habitat preference of this species. While other costs and benefits may be associated with the symbiosis of scallops and their epibionts, epibiotic coverage does not confer any protection from predation, and so in the absence of further information the relationship should be classified as commensalistic.