A study of the taxonomy and distribution of hermatypic corals of the chagos archipelago, Indian Ocean

This study is based on corals collected during the Joint Services Expedition to Egmont Atoli (in 1972) and to the western part of the Chagos Bank (In 1975). The corals were identified to generic level, and where appropriate to species and species groups. A systematic check-list is presented covering the material from both collections. Much of the Information from Egmont has been lost, especially regarding the locations where specimens were collected. It was hoped however to use the data from the Chagos Bank collection to classify the coral communities In terms of associations, according to the methodology of the Zurich-Montpeiller School of Sociology (Braun-Blanquet, 1951). This was attempted with the aid of suitable computer analysis, but owing to the highly variable nature of the data, the approach had to be abandoned. Nevertheless, some patterns may be observed in the distribution of the corals from Chagos Bank, principally in relation to depth. No definite zones can be delimited, but some of the genera and species are more conspicuous at certain depths. This is most obvious in the shallowest collections (3 - 6m) and the deeper areas (33 - 45m). The major factors Influencing local distribution patterns of hermatypic corals are discussed, and the Chagos Bank data are compared with similar studies in the Indo-Pacific. At least 55 hermatypic genera have so far been recorded from the Chagos Archipelago. There is a high diversity of coral genera in the western Indian Ocean, which may or may not be continuous with the very rich Indonesian-west Pacific area. The records presented here suggest that the high diversity region In the Southern Maldives may extend south of the equator to include the Chagos reefs. Few quantitative studies have been carried out on reef corals, and most of the work has been confined to the accessible reef flat, while deeper areas of the reef front have been neglected. Furthermore, sampling methods and the kind of Information recorded have varied greatly. It Is recommended that a standard sampling procedure Is laid down, so that data collected in the course of future studies may be meaningfully compared.