Coral reefs, and their associated systems of mangroves and seagrasses, are the world's most biologically diverse marine ecosystems. Important assets to local and national economies, they provide fisheries for food, materials for new medicines, and income from tourism and recreation, as well as protect coastal communities from storms.
Coral reefs are some of the oldest ecosystems on earth.
Built by a variety of coral and other species that secrete skeletons of calcium carbonate that accumulate over time, coral reefs can grow to be massive structures extending over wide areas of shallow tropical and subtropical seas. Reef building corals contain tiny cells of symbiotic algae that convert sunlight and nutrients into fuel for coral growth and production. Because they depend on light, coral reefs require clear water for growth and can be severely damaged by sediment or other factors that reduce water clarity or quality. Other types of corals that do not require warm water or sunlight are found in deep water, providing important habitats for commercial, recreational and other species. A coral head is a colony of small animals called polyps.