Coastal habitats
Littoral zoneIntertidal zoneEstuariesMangrove forestsSeagrass meadowsKelp forestsCoral reefsContinental shelfNeritic zoneOcean surface
Surface microlayerEpipelagic zoneOpen ocean
Pelagic zoneOceanic zoneSea floor
SeamountsHydrothermal ventsCold seepsDemersal zoneBenthic zoneMarine sedimentvteA coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny and agitated water. Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician, displacing the microbial and sponge reefs of the Cambrian.
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