Zoantharians may be either solitary or colonial. Extinct colonial corals. Although various types of corals can be found from the water's surface to depths of 19,700 ft. Because reef- building corals have a symbiotic relationship with a type of microscopic algae, sunlight is necessary for these corals to thrive and grow. Clear water allows light to reach the symbiotic algae living within the coral polyp's tissue.
Many scientists believe that the algae, called zooxanthellae, promote polyp calcification.
See symbiosis for more information on this algae and its relationship with coral. Warm water flows along the eastern shores of major land masses. Reef development is generally more abundant in areas that are subject to strong wave action. Precipitation of calcium from the water is necessary to form a coral polyp's skeleton. This precipitation occurs when water temperature and salinity are high and carbon dioxide concentrations are low.