SPATIAL VARIATION IN THE FAUNA ASSOCIATED WITH MYTILUS-EDULIS ON A WAVE-EXPOSED ROCKY SHORE

The fauna associated with Mytilus edulis at a rocky wave-exposed local ity in North Wales is reported. A total of fifty-nine taxa with repres entatives from most of the main invertebrate phyla was recorded. In te rms of species richness the community was dominated by crustaceans (25 taxa), molluscs (10 taxa) and chelicerate arthropods (9 taxa), mainly mites. Foraminiferans and crustaceans were the most abundant taxa, re presenting 25% and 23% of the total associated fauna respectively. Nem erteans and nematodes were also well represented. The mussel bed was b roadly stratified into three distinct layers each dominated by a singl e species. Thus, the foraminiferan Ammonia batavus was restricted to t he sediment in the bottom layer, the barnacle Semibalanus balanoides o ccurred only on the outer exposed surfaces of the mussel shells, whils t the tiny brooding bivalve, Lasaea rubra, nestled mainly amongst the complex matrix of byssal threads and shell fragments in the middle lay er of the bed. Species richness and diversity decreased systematically with tidal elevation and both these indices were also significantly d epressed amongst mussel communities from highshore tide pools. Inclina tion of the rock surface had little or no effect on the mussel populat ions or their associated fauna. Small mussel patches generally support ed fewer taxa per unit area and had a lower diversity than larger, mor e extensive patches. These marked, small-scale spatial variations exhi bited by the fauna associated with M. edulis appear to be related larg ely to the degree of aerial and tidal exposure, mussel density, the am ount of accumulated sediment and mussel patch size.