The corals that form the structure of the great reef ecosystems of tropical seas depend on symbiotic photosynthesizing unicellular algae called 'zooxanthellae' that live within their tissues.
Other reef creatures have symbiotic zooxanthellae, which they may also expel under stressful conditions. Bleaching stress is also exhibited by soft corals, giant Tridacna clams and some sponges.
The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed.
Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles.