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Branch tips from colonies of a staghorn coral Acropora formosa were exposed to elevated concentrations of copper in order to examine the potential of the'bleaching'response as a means of assessing stress in corals. Two identical toxicity tests were conducted over 48 h using 2 colonies with naturally different algal (zooxanthellae) densities. The tlps from both colonies bleached in proportion to the copper concentratlons, through loss of zooxanthellae to the surrounding water A higher incidence of mortality was observed In tips from the colony with fewer zooxanthellae. During the tests, control corals lost zooxanthellae at a rate of c1% of the algal standing stock d-', zooxanthellae release from both control and experimental corals occurred 2 x faster dur~ ng the daytime than night time, and the number of zooxanthellae appearing as doublets (the mitotic index) was 5 to 8x higher in zooxanthellae released from the …