Coral Reefs

Coral reefs are aragonite structures produced by living organisms, found in marine waters with little to no nutrients in the water. High nutrient levels such as those found in runoff from agricultural areas can harm the reef by encouraging the growth of algae. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonates. The accumulation of skeletal material, broken and piled up by wave action and bioroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although 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, which is also 165 feet.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 degree celcius. However, deep water corals can exist at greater depths and colder temperatures. Although they also form reefs, very little is known about them.
Adapted from: Wikipedia (search CORAL REEFS)