Corals are found throughout the oceans, from deep, cold waters to shallow, tropical waters.
Based on current estimates, shallow water coral reefs occupy approximately 284,300 square kilometers (110,000 square miles) of the sea floor. [a] If all of the world's shallow water coral reefs were placed side-by-side, they would occupy an area a bit larger than the state of Texas. This area represents less than 0.015 percent of the ocean. Yet coral reefs harbor more than one quarter of the ocean's biodiversity. No other ecosystem occupies such a limited area with more life forms. For this reason, reefs are often compared to rainforests, which are the only other ecosystem that can boast anywhere near the amount of biodiversity found on a reef. Coral reefs are sometimes called rainforests of the seas. [b]
Shallow coral reefs grow best in warm water (70–85° F or 21–29° C). It is possible for soft corals to grow in places with warmer or colder water, but growth rates in these types of conditions are very slow. Reef-building corals prefer clear and shallow water, where lots of sunlight filters through to their symbiotic algae. It is possible to find coral reefs at depths exceeding 91 m (300 ft), but reef-building corals generally grow best at depths shallower than 70 m (230 ft). The most prolific reefs occupy depths of 18–27 m (60–90 ft), though many of these shallow reefs have been degraded. However, mesophotic coral ecosystems, where the dominant habitat-forming species can be comprised of coral, sponge, and algae species, are found a depths between 30 and 150 m in tropical and subtropical waters. Corals also need salt water to survive (between 32 to 42 parts per thousand), so they also grow poorly near river openings with fresh water runoff. [c] Other factors influencing coral distribution are availability of hard-bottom substrate, the availability of food such as plankton, and the presence of species that help control macroalgae, like urchins and herbivorous fish.
ReefBase, a global information system for coral reefs, maintains a series of ReefGIS maps that show the location of coral reefs around the world.
Deep-sea coral communities thrive on continental shelves and slopes around the world, sometimes thousands of meters below the ocean surface. Unlike the well-studied tropical coral reefs, these corals inhabit deeper waters on continental shelves, slopes, canyons, and seamounts in waters ranging from 50 m to over 3,000 m in depth. A few species also extend into shallower, cold waters in the northern latitudes. [d] Deep-sea corals are found in all oceans, including the Subantarctic. [e] Like their shallow-dwelling relatives, deep-sea corals exhibit high biodiversity.
Deep-sea coral habitats appear to be much more extensive and important than previously known. Currently, it is impossible to ascertain the overall extent of deep coral communities because so many of the deeper areas these communities inhabit have been explored incompletely or have not been explored at all. With recent advances in deep-sea technology, scientists are beginning to locate and map the distribution of deep-sea coral habitat. [d]