How does it occur?
Corals can only accept a small change in the environment before they get stressed. When a coral feels stressed they expel between 60-90% of their zooxanthellae from their polyps and the zooxanthellae typically lose between 50-80% of their photosynthetic pigments making the corals appear white, or transparent. As you can see in the above picture different parts of corals can have different levels of vulnerability which is why corals can appear to be partially white and partially coloured. This loss of zooxanthellae means the corals loose their source of oxygen and are in a state of starvation meaning they can not lay down their calcium carbonate skeletons and instead use the energy they can obtain to focus on living. The corals will usually retrieve the symbiotic algae over time, however if the stress continues or gets worse the empty polyps will die causing the coral to waste away.
Some scientists believe that this method of zooxanthellae expulsion is a way of adapting to the changes of the environment, getting rid of the algae that can't cope in hope to obtain better adapted free swimming algae. This is a very high risk adaption the corals have adopted in an attempt to better adapt to the climatic changes.
Some scientists believe that this method of zooxanthellae expulsion is a way of adapting to the changes of the environment, getting rid of the algae that can't cope in hope to obtain better adapted free swimming algae. This is a very high risk adaption the corals have adopted in an attempt to better adapt to the climatic changes.
Picture References
Wikipedia. (16.11.12). Coral Bleaching . Available: http://en.wikipedia.org/wiki/Coral_bleaching. Last accessed 6.12.2012.
GBRMPA. What Causes Coral Bleaching . Available: http://www.gbrmpa.gov.au/outlook-for-the-reef/climate-change/what-does-this-mean-for-species/corals/what-is-coral-bleaching. Last accessed 6.12.2012.
Wikipedia. (16.11.12). Coral Bleaching . Available: http://en.wikipedia.org/wiki/Coral_bleaching. Last accessed 6.12.2012.
GBRMPA. What Causes Coral Bleaching . Available: http://www.gbrmpa.gov.au/outlook-for-the-reef/climate-change/what-does-this-mean-for-species/corals/what-is-coral-bleaching. Last accessed 6.12.2012.