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Hard corals Each no bigger than a fingernail, hard corals build reefs by growing atop the stony skeletons of previous coral colonies. Assuming such shapes as plates, domes, and branches, coral colonies have created the largest structures ever made by living things. But like any masterpiece, a coral reef takes time, growing about a half inch (1.3 centimeters) a year. Armed with tentacles that help them “fish” for meals of minute plankton, the individual corals, or polyps, are tubelike animals related to jellyfish and sea anemones. Unlike most of their cousins, though, corals harbor their own built-in food factories.Inside corals' clear outer tissues live microscopic algae, which transform sunlight into sugars through photosynthesis. The hosts help themselves to some of the sugars and even gain a bit of added color.