Grazing by Diadema antillarum (Philippi) upon algal communities on rocky substrates

Grazing by Diadema antillarum sea urchins is well documented for coral reefs, although information is scarce for the eastern part of the Atlantic Ocean, where rocky substrate dominates the sea bottom. This study analysed grazing activity by D. antillarum upon the algal communities living on rock substrates, and its possible impact on the subtidal communities. Controlled feeding experiments using exclusion cages, were performed between May and September 1998 at Madeira Island (NE Atlantic). Three experimental treatments were used: (1) closed cages to exclude sea urchins, (2) open cage controls, and (3) uncaged controls (nine replicates in each treatment). After four months, in September 1998, the percentage algal cover was quantified. One-way analysis of variance followed by Post Hoc Tukey (HSD) tests showed significant differences between algal abundance in the presence (uncaged and open cage controls) or absence (closed cages) of sea urchins. In the areas where D. antillarum was excluded (closed cages), algal abundance increased by about 10% in the four-month period. Diadema antillarum thus effectively reduces algal abundance, and this may have important consequences in determining the algal community structure of rocky substrate.