Enlarge / This southern reef escaped bleaching, but other areas were not so lucky. (credit: Rick Stuart-Smith, Reef Life Survey) In a heat wave, humans sweat and try to avoid heat exhaustion. But in a coral reef, a wave of warm ocean temperatures can trigger a very different response from ours. Corals host single-celled, photosynthetic symbiotes called zooxanthellae that provide food in exchange for shelter.