Bleached brain coral (order Scleractinia). When corals are stressed by changes in conditions, such as water temperature or sunlight, they expel the symbiotic algae (zooxanthellae) living in their tissues, causing them to lose their colour or 'bleach'. Bleached coral is not necessarily dead and may recover if the stressor is removed in time, but the bleached coral is highly susceptible to disease and mortality is high. Thousands of marine animals depend on coral reefs for survival. The leading cause of coral bleaching is rising ocean temperatures due to climate change.