At high temperatures the amount of oxygen produced by the symbiotic photosynthetic organisms becomes toxic to the coral animals. Coral bleaching is a recent phenomenon, the cause of which is not clear,although it appears to be related to high temperatures. Most reefs are found between 5-15 degrees latitude north and south. The reefs are contructed by mutualistic cnidarians in association with photosyntheticprotists. The calsium in the coral is secreted by the coral animals.
Reefs are among the most complex and diverse biological communities in theworld.
Fish that live in the reefs are adapted to habitats that require precise control over movement, versus the premium on speed and stream-lining found in pelagic fish. In allcases, exposes organisms to extremes of salinity and desiccation.