Assessing the herbivore role of the sea-urchin Echinometra viridis: Keys to determine the structure of communities in disturbed coral reefs.

Echinometra viridis previously was considered a cryptic species unable to control the development and growth of macroalgae on coral reefs. Its role as a herbivore was seen as minor compared to other grazers present on the reef. However, the present disturbed state of some reefs has highlighted the role played by this sea-urchin. Combining field data with experiments on the Caribbean coast of Panama, we demonstrate that the current community organization on disturbed coral reefs in the Mesoamerican Caribbean is largely due to the action of E. viridis. It is the most abundant sea-urchin species, together with two others (Diadema antillarum and Echinometra lucunter). Field data also indicate that the relationship between its density and the abundance of macroalgae is stronger and it is more negative in impact than those of the other two. However, the niche this urchin exploits most efficiently is confined to leeward reefs with low levels of sedimentation. Outside these habitats, their populations are not decisive in controlling macroalgal growth. Grazing experiments showed that E. viridis consumes more fresh macroalgae per day and per weight of sea-urchin, and is a more effective grazer than D. antillarum or E. lucunter. E. viridis showed food preferences for early-successional turf macroalgae (Acanthophora spicifera), avoiding the less palatable late-successional and fleshy macroalgae (Lobophora variegata, Halimeda opuntia). However, it becomes a generalist herbivore feeding on all varieties of macroalgae when resources are scarce. H. opuntia is the macroalga that most resists E. viridis activity, which may explain its wide distribution.