Coral Bleaching

Many of you have probably heard the term coral bleaching before but may not know exactly what it is and how it happens. Corals are a dynamic organism. However, I will attempt to provide you with a general explanation of coral bleaching.

Hard corals (Scleractinain corals) have a mutualistic relationship with zooxanthellae, a symbiotic algae, within the coral polyp's tissues. Images of coral polyps are shown below. Corals can not survive without the symbiotic algae. Zooxanthellae provide up to 95% of the carbon necessary for coral growth, reproduction, and maintenance.

 

 

Environmental stress to corals disrupts the mutualistic relationship between the corals and zooxanthellae. Which results in the expulsion of the symbiotic algae from the coral tissues, a process known as coral bleaching. Expulsion of the zooxanthellae results in death of the coral polyps. Mass mortality due to bleaching events have been linked to increased sea surface temperatures. However, bleaching has also been attributed to other environmental disturbances such as prolonged low temperatures, subaerial exposure, freshwater dilution, changes in turbidity, sedimentation, changes in light levels, UV radiation, and pollutants. The picture shown below was taken on Snapshot Reef in Fernandez Bay, San Salvador on May 30th, 2015.

 

The first couple of days diving on Snapshot Reef we did not encounter any bleached corals. However, on our third day of sampling we found the bleached lobed star coral shown above. Below is an image of live lobed star coral on the same reef.