Density-dependent size regulation and negative growth in the sea urchin Diadema antillarum Philippi

This study documents size- and density-dependent growth (positive and negative), in the sea urchin Diadema antillarum. In the summer of 1983, an inverse relationship was found between Diadema test diameter and population density at seven sites in Lameshur Bay, St. John, United States Virgin Islands. The regression of this relationship improved when test diameter was plotted against density per unit grazing area. A field experiment demonstrated that 1) Diadema has the ability to reduce skeletal body size, and 2) direction (growth or shrinkage) and rate of growth can be predicted accurately based on the urchin's body size and population density. The ability to adjust body size as density fluctuates may allow Diadema to persist when density increases, by shrinking and reducing metabolic costs, and to take advantage of decreases in density, by increasing in size and fecundity.