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Sea urchins are key components of coral reefs. Their feeding activity in association with their abundance has been a focus of reef research in the past three decades. It is well-known that some sea urchins can reach a density of >50 individuals mÀ2 (Glynn,
1988) and exert a strong top-down herbivorous effect on macroal- gae on reefs (Sammarco, 1982; Aronson and Precht, 2000). Only a density of 10 individuals mÀ2 of Diadema antillarum (Philippi) was sufficient to remove all of the daily macroalgal production of 7 g dw mÀ2 on the reefs of the Spanish Canarian Archipelago and maintain the substrate as a barren ground (Tuya et al., 2004a). In addition to herbivory on erect macroalgae, sea urchins feed on tuft algae, endolithic microalgae, barnacles, tube-building polychaetes and juvenile corals. Such feeding activities remove the coral skeleton associated with the algae and cause erosion to the reef structure. Bioerosion of calcium carbonate by sea urchins can reach 40 kg mÀ2 yrÀ1 (Glynn, 1988), which exceeds the typical accretion rates of 0.3 – 12 kg mÀ2 yrÀ1 in tropical reefs (Smith, 1983). There- fore, feeding by sea urchins can be a positive or negative force in coral reefs. At adequate densities, sea urchins can prevent the over-growth of macroalgae, providing space for coral settlement and juvenile growth; whereas at high densities, sea urchins can have a negative effect on the calcium carbonate budget.
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