Patterns of echinoid bioerosion in two Pacific coral reef lagoons

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Abstract
Echinoid populations can be major grazers on hard reef substrata and are consequently of importance in the bioerosion of coral reefs. There are no previous data on rates or patterns of bioerosion by the echinoid community in Indo-Pacific reefs. We surveyed densities of echinoid populations in 2 reef lagoons, the Tiahura section, northwest Moorea (Society Islands) and the central lagoon of Takapoto atoll (Tuamotu Islands), and determined echinoid erosion rate of the reef structure. Echinometra mathaei was the most numerous urchin in Tiahura (7.4 m-2) followed by Diadema savignyi (4.8 m-2), Echinothrix diadema (0.6 m-2) and Echinostrephus molaris (0.2 m-2). E. mathaei and E. molaris are small urchins processing small amounts of carbonate rock and the impact of their populations as bioeroders is limited at such densities, even for the numerous E. mathaei (1.0 g m-2 d-1). D. savignyi and E. diadema are large (mode test diameter 55 and 60 mm respectively) and their populations consumed 9.2 and 2.2 g m-2 d-1 of hard substratum respectively. In Takapoto lagoon, the same species were present (except E. molaris), but densities of urchins were extremely low and echinoid bioerosion was only of importance close to lagoonal inlets (2.6 g m-2 d-1). In situ growth of the hard reef substratum in Moorea is predominantly through growth of massive Porites colonies and estimated at 6 g m-2 d-1. The conclusion is that, with an echinoid bioerosion rate of 12.5 g m-2 d-1 the reef structure is being destroyed. The observed urchin densities are highly for oceanic islands of the Indo-Pacific region and the possibility of a relation between human influence on the reef and urchin densities should be considered.