What is the secret behind the demise of this coral? I don't know for sure, but I can make a pretty good guess based on the picture. Can you?
It looks like this coral has been the victim of that famous corallivore, the crown of thorns starfish (Acanthaster sp.). You can see part of the culprit in the center bottom of the picture. What looks like a patch of grey, red-tipped spines is actually the starfish. The crown of thorns is infamous for eating corals. They crawl on top and digest the fleshy parts, leaving the stripped white skeleton of the coral behind. Their spines are also a painful irritant to humans.
The crown of thorns is distributed throughout the indo-pacific, and has been thought to consist of single species, Acanthaster planci. However recent molecular evidence suggests that there are actually four species of Acanthaster .
This coral-eating starfish can devastate reefs when they appear in high numbers, but there is hope from a coral eating starfish.