Coral bleaching results from the disruption of photosysnthetic microalgae endosymbionts (zooxanthellae), which leads to loss of pigmentation, either from a reduction in zooxanthellae density and/or from decreased concentration of the photosynthetic pigments in the algal cells. Corals may expel their symbiotic algae in response to thermal stress, UV radiation, decrease in salinity, exposure to chemicals, sedimentation, and other environmental factors acting alone or in synergy; bleaching can also be caused by a bacterial infection. Bleached coral reefs offer a patchy appearance due to the irregular distribution of bleached colonies, and variations in the extent of bleaching among individual colonies.
Bleaching may affects the entire colony, upper surfaces , the base, or focal to multifocal patches . Coloration may vary from pale to white within individual species. Severity of bleaching can range from: pale: colonies that are lighter than normal; partially bleached: the loss of some or all color in a random mosaic pattern; mostly bleached: colonies that are uniformly pale yellow to off white; and fully bleached: colonies that appear uniformly white and the skeleton is seen through translucent coral tissue; Other patterns of bleaching also occur including ring bleaching: rings of pale or white tissue up to 4 cm in diameter and 2-5 mm in width, each with a center of unbleached tissue. Bleached colonies may also exhibit a distinct blue, yellow, green, or reddish fluorescence. A scale of bleaching from 1-5 can be used to describe the severity: 1= white; 2= light yellow; 3=yellow; 4= light brown; 5= normal.
It is important to distinguish live tissue that is bleached from white, recently exposed skeleton lacking tissue. Generally, tentacles, polyps, or the mouths of polyps can still be seen in a bleached coral. The surface will also have a layer of mucus, all of which are absent from areas on a colony that are white, but have died.
Corals may recover from bleaching if conditions that promoted bleaching subside. However, prolonged stress can result in progressive mortality. Typically, tissue lysis started in small spots, usually on the verrucae of the branch or at the edge of a focal area of bleaching. Mortality progresses slowly to unite all the white patches by degrading the coral tissue and exposing the bare calcium carbonate skeleton. Often, severe bleaching events are followed by outbreaks of coral disease, especially white syndromes.