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 locality in North Wales is reported. A total of fifty-nine taxa with representatives from most of the main invertebrate phyla was recorded. In terms of species richness the community was dominated by crustaceans (25 taxa), molluscs (10 taxa) and chelicerate arthropods (9 taxa), mainly mites. Fora-miniferans and crustaceans were the most abundant taxa, representing 25% and 23% of the total associated fauna respectively. Nemerteans and nema-todes were also well represented. The mussel bed was broadly stratified into three distinct layers each dominated by a single species. Thus, the foramini-feran Ammonia batavus was restricted to the sediment in the bottom layer, the barnacle Semibalanus balanoides occurred only on the outer exposed surfaces of the mussel shells, whilst the tiny brooding bivalve, Lasaea rubra, nestled mainly amongst the complex matrix of byssal threads and shell fragments in the middle layer of the bed. Species richness and diversity decreased systematically with tidal elevation and both these indices were also signifocantly depressed amongst mussel communities from high-shore tide pools. Inclination of the rock surface had little or no effect on the mussel populations or their associated fauna. Small mussel patches generally supported fewer taxa per unit area and had a lower diversity than larger, more extensive patches. These marked, small-scale spatial variations exhibited by the fauna associated with M. edulis appear to be related largely to the degree of aerial and tidal exposure, mussel density, the amount of accumulated sediment and mussel patch size.