What was done
The authors studied extreme tissue retraction in the scleractinian coral Coeloseris mayeri at coral reefs in Phuket, Thailand.  In particular, they examined the retraction and recovery of coral tissues over a tidal cycle, associated changes in coral color and symbiont algae count, and physiological benefits of tissue retraction.

What was learned
The degree of tissue retraction was related to environmental conditions, "including length of sub-aerial exposure, and intensity of solar irradiance."  Extreme tissue retraction was observed approximately 85 minutes after initial sub-aerial coral exposure.  Tissue retraction, however, did not involve any reduction in chlorophyll concentration or algae symbiont abundance; and the tissues expanded over the coral skeletons to pre-retraction conditions following the return of the tide.  The adaptive benefits of tissue retraction, according to the authors, "include increased albedo, leading to a reduction in absorbed solar energy of 10%, ... and possible avoidance of photochemical damage or photoinhibition at high solar irradiance."

What it means
This study demonstrates the ability of certain corals to cope with the environmental stresses of sub-aerial exposure, including high substrate and air temperatures and solar irradiance.  The authors noted that "tissue retraction in this species results in marked paling of color which has previously been interpreted as a bleaching response."  Hence, it is possible that some reports of coral bleaching, particularly those based solely on satellite reflectance measurements, may not be real bleaching events.