Coral bleaching is the process when stressed corals get rid of the symbiotic algae living in their tissues and turn white. Stressors such as changes in temperature, light or nutrients can cause this color change.

https://oceanservice.noaa.gov/facts/coral_bleach.html

So why use black and white photos to determine coral bleaching sensitivity?

Well, in a recent study, scientists discovered a way to compare bleaching susceptibility in corals using the relative grayscale rather than the more effort-intensive method of zooxanthellae density assessment.

How did they do it?

Coral fragments were sampled from the colonies of five species in southern Taiwan and maintained in the same aquarium tank with circulating seawater and exposed to two different heating regimes. Each fragment was photographed periodically throughout the heating experiment and then the color images were converted to grayscale.  This allowed for digital analysis where the relative grayscales at a particular stage of bleaching were determined and compared.

What did they find?

This showed how close the fragment was to being completely bleached.  (The smaller the relative grayscale, the closer the fragment is to being completely bleached). More sensitive corals bleach earlier than other corals in massive bleaching events and this research indicates that Pocilloporidae are more sensitive than the hydrocoral Millepora spp. Handy to know for those trying to restore reefs and predict future reef health.

For more info: Wang, J.T., Chu, C.W. and Soong, K., 2022. Comparison of the bleaching susceptibility of coral species by using minimal samples of live corals. PeerJ10, p.e12840.

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