Night has fallen in the tropics. Beneath the ocean surface, a soft body and swaying tentacles reach up from a skeletal cup. This is a polyp, the individual of the coral reef. Its tentacles are armed with stinging tips, ready to bring any passing plankton into the body. The polyp is not alone. Its colony of identical swaying polyps may have been around for centuries. Panning further out, the colony is part of a reef that could be thousands of years old and more than a thousand kilometres long. Credit: NOAA /Julie Bedford A generation ago, coral reefs were thought to be immutable. Colonies might occasionally perish at the force of a hurricane or predator, but the overall reefs would carry on, providing a quarter of marine life with food or shelter, providing developing countries with a quarter of their fish catch, providing shorelines with storm protection, and providing jobs in tourism. Then in 2006, two common corals were listed as threatened. In little more than the decade si
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