<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> <P> Most of the coral reefs we can see today were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves . This means that most modern coral reefs are less than 10,000 years old . As communities established themselves on the shelves, the reefs grew upwards, pacing rising sea levels . Reefs that rose too slowly could become drowned reefs . They are covered by so much water that there was insufficient light . Coral reefs are found in the deep sea away from continental shelves, around oceanic islands and as atolls . The vast majority of these islands are volcanic in origin . The few exceptions have tectonic origins where plate movements have lifted the deep ocean floor on the surface . </P> <P> In 1842 in his first monograph, The Structure and Distribution of Coral Reefs, Charles Darwin set out his theory of the formation of atoll reefs, an idea he conceived during the voyage of the Beagle . He theorized uplift and subsidence of the Earth's crust under the oceans formed the atolls . Darwin's theory sets out a sequence of three stages in atoll formation . It starts with a fringing reef forming around an extinct volcanic island as the island and ocean floor subsides . As the subsidence continues, the fringing reef becomes a barrier reef, and ultimately an atoll reef . </P> <Ul> <Li> <P> Darwin's theory starts with a volcanic island which becomes extinct </P> </Li> <Li> <P> As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef . </P> </Li> <Li> <P> As the subsidence continues, the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside . </P> </Li> <Li> <P> Ultimately, the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon . </P> </Li> </Ul>

Who discovered the process of coral reef development