ECHINOID BIOEROSION AS A MAJOR STRUCTURING FORCE OF RED-SEA CORAL-REEFS

Echinoid bioerosion is ecologically important as a limiting factor for reef growth. Diadema setosum and Echinometra mathaei are the most abu ndant sea urchins in the Gulf of Eilat, Red Sea, Bioerosion by these u rchins was estimated from field experiments with urchins placed in und erwater chambers. A novel methodology was developed for measuring the CaCO3 content of fecal pellets and total gut contents. This method is based on measurement of the amount of gas formed by total acidificatio n of a mixed sample. The population density of the urchins was determi ned for the reef flat (depth 0.6-1 m) and the reef slope (depth 6-8 m) . D. setosum, which erodes 310 mg individual(-1) d(-1) is much less ab undant on the reef flat (0. 1 individuals m(-2)) than on the reef slop e (6.4 individuals m(-2)). In contrast, E. mathaei, which erodes 120 m g individual(-1) d(-1), is more dominant on the reef flat (10.5 indivi duals m(-2)) than on the reef slope (3.7 individuals m(-2)). We estima te that echinoid bioerosion converts to carbonate sediments 7%-1% of t he total reef flat calcification and 13%-22% of the total reef slope c alcification. These findings emphasize the importance of echinoid bioe rosion as a structuring force in the coral reef, and its potential imp ortance to the dynamics of reef development. However, they imply that some previous studies may have overestimated the role of echinoid bioe rosion, thus possibly underestimating internal bioerosion.