Spatio-temporal changes in the trophic structure of rocky intertidal mollusc assemblages on a tropical shore

Patterns of the distribution and temporal variation of trophic groups within a benthic mollusc asemblage of a tropical shore (eastern tropical Pacific) were analysed in this study. Four intertidal rocky sites with different wave exposure were studied. A three-factor analysis of variance (fixed factors: trophic groups and level of exposure; random factor: site) and the Student-Newman-Keuls a posteriori test (P = 0.05) were applied to the biomass of trophic groups in order to know the variation among different sites. Carnivores, herbivores and suspension-feeders varied significantly among sites within each exposure level. Suspension-feeders were more abundant at exposed sites, while herbivores dominated in sheltered places. Biomass of herbivores was not correlated to macroalgal biomass. Total biomass was highest at exposed sites, increasing in winter and decreasing in summer. An ordination analysis carried out with the non-metric multidimensional scaling programme (MDS) revealed two trophic groups: group A included samples from exposed sites (E1,E2) and group B included samples from sheltered sites (E3, E4). Graphic methods (ABC curves) detected changes along temporal and spatial scales, and these could be related to a high proportion of small species. Factors such as exposure level and type of substrate may control the distribution of different trophic groups.