SMALL-SCALE AND BROAD-SCALE PATTERNS OF DISTRIBUTION OF THE UPPER-SHORE LITTORINID NODILITTORINA-PYRAMIDALIS IN NEW-SOUTH-WALES

Small-scale (within-shore) and broad-scale (among-shore) patterns of distribution, abundance and size structure of Nodilittorina pyramidalis were measured on a number of shores in New South Wales. This species showed significant patchiness in distribution and size structure. The mean density and mean shell length of snails differed among patches of shore only a few metres apart, among different heights on the shore and from shore to shore. Differences in densities appeared to be strongly correlated to the distribution of particular microhabitats, especially cracks, crevices and pits within the rock surface. This relationship and the presence or absence of these microhabitats appeared to determine the patterns of distribution of this species within and among different shores. The sizes of snails were not correlated with densities of Nodilittorina nor with the coexisting species, Littorina unifasciata, in the populations examined. Processes influencing the distribution and abundance of Nodilittorina were not investigated, but the patterns of distribution found on these shores suggest that recruitment was limited to areas of topographic complexity. It also seems likely that limited dispersal by the adult snails retain them within restricted patches on the shore.