Coral reefs are aragonite structures produced by living organisms, found in marine waters. Corals are found both in temperate and tropical waters, shallow-water reefs are formed only in a zone extending at most from 30°N to 30°S of the equator. Tropical corals do not grow at depths of over 50 m (165 ft). Temperature has less of an effect on the distribution of tropical coral, but it is generally accepted that they do not exist in waters below 18 °C.However, deep water corals can exist at greater depths and colder temperatures. Although deep water corals also form reefs, very little is known about them.
