Excessively warm water can also result in stress to coral colonies by causing bleaching of the polyps.
Corals are host to microscopic algal cells called zooxanthellae, which live within the coral tissue.
The algae photosynthesize, producing oxygen and food from sunlight. These products are used by the coral polyp for respiration and food, and the resultant waste products and carbon dioxide are used, in turn, by the algae. Corals can survive short-term bleaching but are physiologically stressed, increasing their susceptibility to diseases. As temperatures cool from 84F o the polyps, if they survive, are recolonized by algae and the mutual relationship returns to normal. These sensors are called thermographs because they can record water temperature continuously .