Population status of the black sea urchin Diadema antillarum (Philippi) in La Parguera, Puerto Rico, 20 years after the mass mortality event

Before the 1983 mass mortality event, the conspicuous sea urchin Diadema antillarum was considered a key species in Caribbean coral reefs. This epizootic event caused mortalities of up to 99% in many localities of the western Atlantic and Caribbean. In this study, the current status (after 20 years) of populations of D. antillarum was assessed in five reef localities off the southwest coast of Puerto Rico. Densities were estimated by counting all urchins in each of five band transects (20 x 2 m) in each of three depth intervals (0--3 m, 4--10 m, >10 m) at each reef site. Size structure of the urchin populations were estimated by measurement of maximum test diameter of every urchin found along each band transect. Habitat complexity (i.e., rugosity) was determined using the chain method of CARICOMP. Density differences were compared as a function of site, rugosity and depth. A linear regression showed that the densities of D. antillarum where influenced by factors like depth and site (P < 0.0001). The test revealed that in the present state of urchin populations, rugosity did not have a statistically significant effect on densities but a trend to higher densities in more complex areas was observed. Mean densities per reef varied between 0.01--0.82 ind/m2, with a mean density of 0.23 ind/m2 for the entire area surveyed. (Abstract shortened by UMI.)...