A coral ecosystem is a complex marine environment that includes coral reefs, the organisms that inhabit them, and the surrounding ocean. Reefs are formed by colonies of coral polyps, which are small invertebrate animals that secrete calcium carbonate to form a hard skeleton. The REEF provides a habitat for a wide variety of marine life, including fish, mollusks, crustaceans, and other invertebrates. Coral ecosystems are highly productive and provide important services to humans, such as coastal protection, fisheries, and tourism.