Coral bleaching--how and why?

Bleaching refers to the loss of colour in symbioses between dinoflagellate algae of the genus Symbiodinium and marine benthic animals, e.g. corals. Bleaching generally results in depressed growth and increased mortality, and it can be considered as a deleterious physiological response or ailment. An explanatory framework for the causes of bleaching comprises three elements: the external factors or triggers of bleaching, e.g. elevated temperature; the symptoms, including elimination of algal cells and loss of algal pigment; and the mechanisms, which define the response of the symbiosis to the triggers, resulting in the observed symptoms. The extent to which bleaching in different symbioses and in response to different triggers involves common mechanisms is currently unknown, but a contribution of interactions between the algal and animal partners to bleaching is predicted. Symbioses vary in their susceptibility to bleaching as a result of genetic variation in Symbiodinium and acclimatory responses of the animal. The evolutionary explanation for bleaching is obscure. Perhaps, bleaching was of selective advantage to the animal hosts under different (more benign?) environmental conditions than the present, or bleaching may be a negative by-product of an otherwise advantageous symbiotic trait, such as the elimination of damaged algal cells.