scientific chapter |
Abstract French Polynesia is composed of 118 high islands and atolls stretching in a vast oceanic expanse in the central Pacific Ocean, from just below the equator to almost 30° S. It is composed of five archipelagos, which all differ in terms of their geological origin and history, environmental conditions, and reef development. The Tuamotu Archipelago is entirely comprised of atolls, whereas fringing or barrier reefs are found in the Society, Gambier, and Austral Islands (only one atoll), and coral reef accretion is almost nonexistent in the Marquesas. Coral reef studies have concentrated mostly on the Society and Tuamotu Archipelagos, and, except for occasional observations, been restricted to a depth of 30–40 m on the reef slopes. Our knowledge of mesophotic coral ecosystems is therefore limited, particularly for the deeper zone, below 80 m depth. The scleractinian mesophotic fauna is highly diverse compared to shallow reefs, with 38 genera and 96 species recorded. This fact is likely to be a consequence of water clarity, allowing the photophilic shallow coral species to extend deeper than usual. In some areas, substratum cover by Pachyseris speciosa reaches values of 80% at depths of ≥70 m, and a typical “deep” mesophotic assemblage dominated by Leptoseris spp. has also been recognized. Quantitative data on other major biotic components of the mesophotic assemblages are either lacking or insufficient to allow their characterization as depth generalists or depth specialists.