Density, size, and biomass of Diadema mexicanum (Echinoidea) in Eastern Tropical Pacific coral reefs

Diadema is among the most abundant, widely dispersed, and ecologically important genera of sea urchin in tropical shallow waters. D. mexicanum is distributed from the Gulf of California to northern Peru, including the oceanic islands of Revillagigedo, Clipperton, Coco, Malpelo, and Galapagos, and it is one of the most important sea urchin species in Eastern Tropical Pacific (ETP) coral reefs. In the 1980s, El Nino caused high coral mortality, resulting in an increase in macroalgal cover. This resulted in higher sea urchin bioerosion activity, which weakened the reef frameworks. Considering the high vulnerability of the ETP coral reefs, the aim of this study was to determine regional differences in the density, size (test diameter), and biomass of D. mexicanum at 12 localities in 4 countries between 2009 and 2010, and to determine possible causes of these differences. The average density, size, and biomass of D. mexicanum were 0.47 +/- 0.15 ind. m(-2), 4.38 +/- 1.50 cm, and 0.26 +/- 0.33 g m(-2). The test size frequency that predominated was 2 to 3 cm. Predation by macrophagous fishes seems to be one of the most important factors that explains the presence of low densities and small size of sea urchins throughout the region. The increase in D. mexicanum predators is probably a result of overfishing of top predators.