This work was carned out on a fringing reef situated on the leeward coast (North-West) of Reunlon Island (Mascarene Archipelago, Indian Ocean). A boat channel, a compact reef flat and a reef flat with transverse stripes were successively found between shore and reef front. Two structures were defined on the outer reef slope: the coral-built spurs-andgrooves zone (0-20 m) and the lower sloping platform, a volcanlc flagstone with low biogenic coverage (20-30 m). Seawards, thls flagstone is completely covered by a field of free organic nodules. Scleractinlan coral communities were studled with quantitative sampling methods (quadrat and transect techniques) in terms of species composition, zonation and diversity patterns. On the basis of these data, the coral comnlunities were divided into 3 assemblages: ( I ) A community of the boat channel and the compact reef flat. The emersion of this reef flat at low tide induces the development of facies formed by Acropora pharaonis and Pavona divaricata. The reef flat with transverse stripes appears as a transition zone with the second community. (2) A community of the spurs-and-grooves zone of the outer reef slope, divided into 3 subzones. (a) upper level, influenced by strong wave action, characterized by spatlal competition between scleractinian corals and encrusting calcareous algae; (b) intermediate level dominated by a small number of Acropond and Poritid species; (c) lower level, where the decrease in llght intensity reduces the building activity of Scleractinia. (3) A community of the lower part of the outer slope, extending to the nodules field, characterized by settlement of an important number of deep species and by high specles richness. 42 genera and 120 species of Scleractinia reveal a remarkable faunistic richness for the reef studied.
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Claude Bouchon (1981) studied this question.