Population processes in the ribbed mussel Geukensia demissa (Dillwyn) in a North Carolina salt marsh tidal gradient: spatial pattern, predation, growth and mortality

Abstract

Empirical research and field experiments during summer and winter months on ecological processes of a North Carolina population of the rib d mussel, Geukensia demissa (Dillwyn), revealed both similarities and contrasts with previous research on this and related marine bivalves. Growth and mortality patterns along the salt marsh tidal gradient were generally consistent with other studies, although summer epibenthic predation mortality was more intensive and involved animals in larger size classes. Survivorship of all size classes was significantly higher in the low tide zone than the high tide zone, but varying densities had little effect on survivorship. In the summer clump-size experiment, predation mortality in open uncaged plots was extensive. Winter survivorship of small and medium-sized mussels increased with decreasing clump size, especially in the low tide zone, and suggested additional advantages of clumping over that of a refuge from crab predation during summer. Mussel growth was greater, but survivorship lower, in the low marsh zone where predation by crabs during summer months was also greatest. Increased longevity and thus a size structure skewed to larger animals characterized the high marsh zone. During summer, growth rate in the low zone was inversely related to increasing clump size, but the effect disappeared during the winter months. In comparing sizes of mussel vulnerable to crab predation between New England and North Carolina marshes, we suggest that mussels in the low tidal zones of more southerly marshes are less likely to escape predation through the growth process. Even though general patterns of G. demissa intertidal size structures and abundance among east coast marshes are emerging, we suggest that the mechanisms accounting for differential growth and mortality, including possible genotype-dependent processes, are still poorly understood in spite of the rather extensive research on this bivalve group.