Corals are the living part of a coral reef. The majority of reefs are formed by a type of coral called a stony coral. Polyps of stony coral join together to start the beginning of a reef.
The polyps produce calcium carbonate, which turns into a hard exoskeleton that provides protection and support for the polyp's body. As a polyp grows upward it continues to secrete calcium carbonate, which results in the reef growing. Each polyp links itself with other polyps to form the reef. Most of a coral reef is actually dead skeletal material. The top layer is usually the only living part of the structure.
Coralline algae are also important parts of a coral reef. This alga deposits limestone in layers over the reef, which strengthens it. Without these algae, the forces of the waves would quickly break apart a coral reef.
Coral reefs develop mostly in tropical oceans and cover about 1 percent of the surface area of all oceans. The Pacific Ocean, Indian Ocean and Red Sea areas contain over 90 percent of all the world's coral reefs. The ideal water temperature for coral reefs is between 77 and 82 degrees Fahrenheit. This narrow temperature range creates a large problem for reefs, because as the temperatures of the oceans increase the coral has been dying off. In several areas around the world the coral has been destroyed because of the rising ocean temperatures.
Coral reefs grow very slowly. On average they grow about one inch per year horizontally and between one inch to one foot vertically. They need sunlight to grow, so they typically occur in shallow water not more than 450 feet deep. The coral that actually builds the reef is usually not found in water deeper than 150 feet. Reefs do not grow above sea level, so the highest point of a reef is typically below the low-tide mark.
