Spatial variations in density and size of the echinoid Diadema antillarum Philippi on some Venezuelan coral reefs

The distribution, population density and size structure of Diadema antillarum Philippi was found to vary with reef locality, food availability and the structural complexity of the reef. Structural complexity was classified according to the growth morphology and abundance of the coral species found in the different reef zones. Mean urchin density in the reef varied between 2.6 and 3.7 ind. m¯². High densities (4.0-7.0 ind. m¯²) were found in the upper reef zone, which was characterized by high structural complexity, and by a dominance of the coral species Acropora palmata, A. cervicornis, Montastrea annularis and the hydrozoans Millepora alcicornis and M. complanata. Low densities (1.5-4.0 ind. m¯²) were found in the deep fore reef and flat reef platform, which were characterized by low structural complexity. A significant and positive correlation was found between urchin density and the cover of dead substrate.