CORAL BLEACHING - CAUSES AND CONSEQUENCES

It has been over 10 years since the phenomenon of extensive coral blea ching was first described. In most cases bleaching has been attributed to elevated temperature, but other instances involving high solar irr adiance, and sometimes disease, have also been documented. It is timel y, in view of our concern about worldwide reef condition, to review kn owledge of physical and biological factors involved in bleaching, the mechanisms of zooxanthellae and pigment loss, and the ecological conse quences for coral communities. Here we evaluate recently acquired data on temperature and irradiance-induced bleaching, including long-term data sets which suggest that repeated bleaching events may be the cons equence of a steadily rising background sea temperature that will in t he future expose corals to an increasingly hostile environment. Cellul ar mechanisms of bleaching involve a variety of processes that include the degeneration of zooxanthellae in situ, release of zooxanthellae f rom mesenterial filaments and release of algae within host cells which become detached from the endoderm. Photo-protective defences (particu larly carotenoid pigments) in zooxanthellae are likely to play an impo rtant role in limiting the bleaching response which is probably elicit ed by a combination of elevated temperature and irradiance in the fiel d. The ability of corals to respond adaptively to recurrent bleaching episodes is not known, but preliminary evidence suggests that phenotyp ic responses of both corals and zooxanthellae may be significant.