Coral bleaching happens when coral polyps, the animals that build corals, shed the algae (zooxanthellae) that give them their color, and which are necessary for their survival. The coral is then white in appearance.
Coral scientists are not sure what causes coral bleaching, but warming water is the most likely culprit. Corals in the Caribbean and Florida have bleached when sea surface temperatures rose and were higher than the mean sea surface temperature for as little as one month.
Coral reefs are important as shelter and feeding grounds for a variety of fish species. When there is a massive die-off of coral, the bleached coral is colonized by algae, the reef becomes populated by only herbivorous fish, and the reef may never recover.