Density‐Dependent Size Regulation in Diadema Antillarum: Effects on Fecundity and Survivorship

Food limitation can often result in reduced fecundity or death. However, a reduction in body size can minimize these negative effects. This study demonstrates that the sea urchin Diadema antillarum has the ability to adjust body size quickly (by positive or negative growth) to a resource— or density—dependent level. In a field experiment, small (27.5 mm test diameter) and large (37.5 mm) urchins were placed in cages at low (12 individuals/m 2 ) and high (24 individuals/m 2 ) densities. After 26 wk, body size in the high density treatment had converged at 30 mm, and body size in the low density treatment had converged at 36 mm test diameter. Mortality was not size or density dependent. After 32 wk, the effects of initial size on mass, gonad volume, and gamete production could not be detected. Diadema adjusted body size to a density—dependent level predicted by the inverse relationship between mean size and density seen in field populations. In a laboratory experiment, urchins were placed in groups of one or three, and fed one of four rations of food. After 23 wk, the urchins had adjusted test diameter, mass, and gonad volume to a level determined by per capital food availability and not crowding. Gonad volume was not significantly different from that predicted by body size (compared to field populations), regardless of whether the urchins had grown or shrunk. Among the fed urchins, mortality was not resource dependent, even though the urchins in the low—food group exhibited significant negative growth. Size regulation thus increases survivorship and reproductive output under rapidly changing conditions of population density and resource availability. A consequence of this life history strategy is that populations tend to track carrying capacity closely.