Flexi Says: Coral reefs look like big rocks, but they are actually alive. They are built from cnidarians called corals. The corals are sessile (non-moving) polyps that use their tentacles to feed on tiny ocean creatures that pass by. Their skeletons are made up of calcium carbonate, which is also known as limestone. Over long periods of time, their skeletons build on each other to produce large structures known as coral reefs. Coral reefs are important habitats for many different types of ocean life.