Lugo-Ascorbe, Miguel A.

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    Population status of the black sea urchin Diadema antillarum (philippi) in la Parguera, Puerto Rico, 20 years after the mass mortality event
    (2004) Lugo-Ascorbe, Miguel A.; Weil, Ernesto F.; College of Arts and Sciences - Sciences; Aponte, Nilda E.; Yoshioka, Paul; Department of Marine Sciences; Valdés Pizzini, Manuel
    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. A significant decrease in densities of D. antillarum with depth was found. Size distributions varied between depth intervals (K-S, P<0.05) and amongst reefs in some occasions, with higher densities of medium-big urchins in shallow waters and higher densities of small individuals in deeper areas. Reproductive dynamics was assessed using the gonad index (GI= 100 x weight of the gonad divided by the weight of the urchin). Urchins were dissected every month for a period of fourteen months. An increase in GI indicates active gametogenesis and a decrease that urchins spawned. D. antillarum in La Parguera reproduced all yearlong with a reproductive peak between the Spring and Summer in 2001. A correlation between sea surface temperature and GI indicates a possible influence of this factor on the reproductive biology of these urchins. GI started to increase in the fall and the highest proportion of spawning urchins and decreasing GI was observed right after the sea surface temperature began to increase in the Spring following the winter lows. Minimum reproductive size (smallest sexually mature individual) was 3.8 cm in diameter. Sex ratio (♂/♀) was 0.89:1. Overall, it appears that D. antillarum are slowly coming back and populations show high variability in their distribution, densities and size structures within and across different reef localities.