A bleached coral, Acropora sp., taken at Ant Atoll, Pohnpei, Micronesia. Corals are extremely sensitive to temperature changes. Increased water temperatures, which may be linked to global warming, can cause mass coral bleaching. Bleaching occurs when coral polyps, stressed by heat or ultraviolet radiation, expel the symbiotic algae that live within coral tissues. When the algae are expelled,the coral appears white or “bleached.” These algae provide corals with most of their food and oxygen. Corals can recover after short periods of bleaching, but as the length and severity of the stress increase so does coral mortality. Coral bleaching events and subsequent reef mortality are expected to become more frequent as sea temperature increases.