![]() They are most commonly found at shallow depths in tropical waters, but deep water and cold water coral reefs exist on smaller scales in other areas.Ĭoral reefs have declined by 50% since 1950, partly because they are sensitive to water conditions. ![]() Coral reefs flourish in ocean waters that provide few nutrients. They occupy less than 0.1% of the world's ocean area, 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. Sometimes called rainforests of the sea, shallow coral reefs form some of Earth's most diverse ecosystems. Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician, displacing the microbial and sponge reefs of the Cambrian. Most reefs grow best in warm, shallow, clear, sunny and agitated water. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most coral reefs are built from stony corals, whose polyps cluster in groups.Ĭoral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish. Reefs are formed of colonies of coral polyps held together by calcium carbonate. ![]() A coral reef is an underwater ecosystem characterized by reef-building corals. ![]()
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