Gastropod Abundance and Biomass Relationships with Salt Marsh Vegetation within Ocean-dominated South Carolina, Usa Estuaries

Molluscs, both gastropods and bivalves, appear to disproportionately influence the dynamics of salt marsh habitats. Bivalves typically are facilitators positively affecting the growth and survival of marshes, but gastropods may have unanticipated top-down effects through direct consumption of the predominant marsh vegetation, Spartina alterniflora. Natural, elevational differences in S. alterniflora density and morphology also may exert an influence on the spatial distributions of marsh snails. We examined the abundance of both plants and snails across an elevation gradient to determine if similarities or differences existed in the observed distribution patterns. Plant, mainly S. alterniflora, and snail, Littoraria irrorata (Say) and Melampus bidentatus Say, densities were measured along tidal creek to forest transects within the intertidal marshes of three marine-dominated South Carolina inlets. Significant differences in S. alterniflora density among marsh zones were paralleled by similar L. irrorata density differences. A consistent, unimodal pattern with peak L. irrorata density in the “short,” dense S. alterniflora mid marsh occurred within each inlet. In contrast, M. bidentatus was restricted almost entirely to the high marsh across all transects and inlets. Densities of L. irrorata and S. alterniflora were positively correlated in the low marsh consistent with the potential importance of culms as a food source and refuge from predators. Littoraria irrorata biomass was significantly different among zones in only one inlet and increased in a shoreward direction in two inlets. Although biomass patterns previously have not been reported for Southeastern US marshes, amounts were consistently greater than expected within the high-marsh zone. In SC, L. irrorata appears to be a conspicuous resident across the entire vegetated marsh and our results suggest previous studies focusing on the marsh near tidal creeks could underestimate appreciably the total population and overall significance of L. irrorata within coastal marshes.