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An updated list of over 200 species of corals from Cosmoledo and Aldabra atolls is presented, which more than doubles previously known species diversity, and establishes these atolls as amongst the most species-rich in the Western Indian Ocean. However, partly this is an artefact of a new method of recording with digital cameras, described here, which greatly improves recording efficiency. This is the first underwater study of Cosmoledo, and the first for Aldabra outside the expedition reports cited. The survey extended to >30 m depth, and comes after the 1998 massive coral mortality. Coral cover was virtually eliminated at that time to about 8-10 m depth in Cosmoledo on seaward slopes, below which coral mortality was only about 50%. Mortality was selective regarding different species, genera and families. Cosmoledo's lagoon of >150 km 2 is shallower than the 'critical depth' of 8-10 m, resulting in an almost complete elimination of corals. To compare these atolls with other reefs in the region, critical depths are summarized for over 25 Indian Ocean locations. New coral recruits are abundant in the shallows of Cosmoledo and Aldabra 4 years later, though cover remains very low. Much bare rock remains (with turf algae) and some genera such as Acropora , previously apparently abundant, remain relatively very scarce. Apart from Porites , whose higher survival is now well documented, the best survivors from the 1998 mortality, and the most successful recruitment of new corals, are of faviids. Soft corals remain extremely scarce in all locations examined above the 'critical depth'. It is predicted that there may be a shift in the identity of the main species of corals in these atolls for many years. [ABSTRACT FROM AUTHOR]