<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> Coral reefs are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals . Coral reefs are built by colonies of tiny animals found in marine water that contain few nutrients . Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups . The polyps belong to a group of animals known as Cnidaria, which also includes sea anemones and jellyfish . Unlike sea anemones, corals secrete hard carbonate exoskeletons which support and protect the coral polyps . Most reefs grow best in warm, shallow, clear, sunny and agitated water . </P> <P> Often called "rainforests of the sea", shallow coral reefs form some of the most diverse ecosystems on Earth . They occupy less than 0.1% of the world's ocean surface, about half the area of France, yet they provide a home for at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians . Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients . They are most commonly found at shallow depths in tropical waters, but deep water and cold water corals also exist on smaller scales in other areas . </P> <P> Coral reefs deliver ecosystem services to tourism, fisheries and shoreline protection . The annual global economic value of coral reefs is estimated between US $30--375 billion . However, coral reefs are fragile ecosystems, partly because they are very sensitive to water temperature . They are under threat from climate change, oceanic acidification, blast fishing, cyanide fishing for aquarium fish, sunscreen use, overuse of reef resources, and harmful land - use practices, including urban and agricultural runoff and water pollution, which can harm reefs by encouraging excess algal growth . </P>

Where would a coral reef most likely be found