Clam predation by whelks (Busycon spp.): Experimental tests of the importance of prey size,prey density,and seagrass cover

In 57 l-m² samples within a meadow of Halodule wrightii in Bogue Sound, North Carolina, USA, densities of the clams Mercenaria mercenaria and Chione cancellata were positively associated with seagrass cover. Where seagrass was experimentally removed, marked individuals of both clam species exhibited high rates of mortality in fine sand sediments during two successive experiments spanning 13 months. In the unaltered (control) seagrass meadow, M. mercenaria density remained constant over 13 months and C. cancellata density declined at a slower rate than in the unvegetated plots. Seagrass provides these clams with a refuge from whelk (Busycon carica, B. contrarium, and B. canaliculatum) predation, the major cause of mortality and population decline in experimentally unvegetated plots. In 2 factorial field experiments in unvegetated substratum in which densities of M. mercenaria and C. cancellata were varied independently, first over 5 levels (0 X, 1/2X, 1 X, 2 X, 4 X) and subsequently over 4 levels (0 X, 1/4 X, 1 X, 4 X), there was no repeatable intra- or interspecific effect of density on percent survival, or on the rate of any mortality type. Whelk predation fell preferentially on larger size classes of both species, whereas factors which contribute to clam disappearance usually acted more intensely on smaller sizes. Experimental exclusion of large predators by caging demonstrated that even in unvegetated substratum survivorship of both clam species was high in the absence of whelks and other predators. Individuals of C. cancellata live closer to the sediment surface than those of M. mercenaria, which may explain why seagrass does not serve as effectively to protect them from whelk predation. The mechanism of whelk inhibition may depend upon sediment binding by the H. wrightii root mat, which produces a demonstrable decrease in the physical penetrability of surface sediments.