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 withi n a benthic mollusc asemblage of a tropical shore (eastern tropical Pacific ) were analysed in this study. Four intertidal rocky sites with different w ave exposure were studied. A three-factor analysis of variance (fixed facto rs: trophic groups and level of exposure; random factor: site) and the Stud ent-Newman-Keuls a posteriori test (P = 0.05) were applied to the biomass o f trophic groups in order to know the variation among different sites. Carn ivores, herbivores and suspension-feeders varied significantly among sites within each exposure level. Suspension-feeders were more abundant at expose d sites, while herbivores dominated in sheltered places. Biomass of herbivo res 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 programm e (MDS) revealed two trophic groups: group A included samples from exposed sites (E1,E2) and group B included samples from sheltered sites (E3, E4). G raphic methods (ABC curves) detected changes along temporal and spatial sca les, and these could be related to a high proportion of small species. Fact ors such as exposure level and type of substrate may control the distributi on of different trophic groups.