MEPS 125:137-147 (1995)  -  doi:10.3354/meps125137

Patterns of abundance and spatial dispersion were analysed in an assemblage of polychaetes associated with mussel beds in a wave-exposed rocky intertidal habitat on the Pacific coast of subtropical South America. Mussel beds were made up of 2 zones in the intertidal, the lower, more extensive zone formed by Semimytilus algosus and the upper by Perumytilus purpuratus. Total polychaete density in the Semimytilus habitat was in the range of 18800 to 36300 ind. m^-2, comparable to some high values recorded for polychaete communities in soft sediments. This indicates that mussel-bed environments on hard substrata are not a marginal but major habitat for polychaetes (free-ranging forms in particular) on the predominantly rocky coast of western South America. Populations of 6 taxa (Pseudonereis, Halosydna, Lumbrineris, Scoloplos, Mediomastus and Typosyllis) demonstrated moderate seasonal fluctuations, with numbers generally low in austral winter and reaching maxima in spring/summer. Numerically, Typosyllis was the most abundant taxon, accounting for 52 to 71% of all individuals. All the taxa showed a contagious (aggregated) dispersion, with the deposit-feeder Mediomastus demonstrating the strongest aggregation and largest fluctuations over a year. Another deposit feeder with limited mobility, Scoloplos, also demonstrated relatively strong aggregation. Iwao's m*-m regression analysis revealed that mean crowding measure was linearly related to mean density in Pseudonereis, Halosydna, Lumbrineris and Scoloplos, with the highest value of slope beta (2.06) for Scoloplos, while no relation was evident in Mediomastus and Typosyllis. Density was generally lower in the upper Perumytilus habitat, in particular for the 2 free-ranging taxa Halosydna and Typosyllis, while the free-ranger Syllis had a higher density in the Perumytilus than in the Semimytilus bed. All taxa were more strongly aggregated in the Perumytilus than in the Semimytilus habitat, probably reflecting the variability and more patchy occurrence of favourable microhabitats and available resources in the former. Among the 5 taxa that occurred in abundance in both Semimytilus and Perumytilus habitats, the less-mobile deposit-feeder Scoloplos had the highest degree of contagion indicating its patchy, aggregated distribution patterns, while the predaceous, completely free-ranging Halosydna demonstrated a relatively weak aggregation. Overall, the lack of regular dispersion patterns and the consistent occurrence of contagious distributions suggested that polychaetes associated with mussel beds did not demonstrate territorially based interactions as seen in some sessile, tube-dwelling species. Complex, coarse-grained habitat structures of mussel beds support high abundances and species richness of polychaetes, allowing the expression of taxa-specific patterns of dispersion as a result of interactions beween behaviours and environmental conditions.

Polychaetes . Abundance . Dispersion patterns . Mussel beds . Rocky shores