These coral animals begin life as free-floating larvae, but settle on the sea floor in sedentary colonies. The term "coral" applies both to these animals and to their skeletons, particularly the skeletons of stone-like corals.
Many different organisms--including mollusks, sponges, and worms--help shape reefs, but hard corals and various algae are the major architects. In effect, the corals build limestone, because their skeletons are made of calcium carbonate. The skeletons deposited by these corals and other organisms accumulate, along with sand and other debris, to form the backbone of the reef. Over tens of thousands of years, chemical and mechanical changes turn the reef into true rock. pull quote: Coral reefs are the only living structure that can be seen from outer space.