Bioerosion in coral reef communities in southwest Puerto Rico by the sea urchin Echinometra viridis

Bioerosion is one of the most important structuring forces in coral reef communities. The bioerosion impact of several species of fish, sponges and sea urchins have been estimated in the Caribbean; however, there is no information for one important species, the red sea urchin Echinometra viridis. This species can be found in high densities in many localities. In this study, bioerosion rates for E. viridis were estimated in two patch reefs off La Parguera, southwest Puerto Rico, using the population size-class distribution, average densities, and the CaCO ₃ content in fecal pellets produced over 24 h. Average densities of urchins along four depth intervals were estimated using 40-m transect lines and 1-m ² quadrats. Average size and size-structure distribution were estimated by measuring the diameter of 180–220 urchins haphazardly collected at each of the four depth intervals. The ignition–loss method was used to estimate the daily rate of bioerosion. Fecal pellets produced by the urchins over a 24 h period were collected in buckets, rinsed in fresh water, dried for 24 h at 70°C, and then burned in a furnace at 550°C, first to eliminate organics, and then at 1000°C until constant weight to determine the amount of calcium carbonate (CaCO ₃) in the fecal pellets. HCl (10%) was then added to the remainder of the sample to test for presence of CaCO ₃. Average individual CaCO ₃ bioerosion rates were estimated at 0.181±0.104 g day ^-1. Average densities (0.77–62.0 ind. m ^-2), size (2.01–2.44 cm) and average bioerosion rates (0.114–4.14 kg m ^-2 year ^-1) were significantly higher in shallow areas (1–3 m) in both reefs. Bioerosion rates were low compared to those reported for parrotfish, endolithic sponges and the black sea urchin D. antillarum, but they were higher than those reported for other small-sized sea urchins in the Caribbean and the Indo-Pacific.