Coral reefs are some of the most impressive natural wonders on our planet. Throughout the world, they are estimated to cover over 284,000 square kilometers, mostly in the Indo-Pacific region. They are produced by living organisms, and provide habitat for a wide variety of other life, such as fishes, marine mammals, and invertebrates.
The formation of a coral reef begins when stony coral larvae, known as polyps, attach themselves to hard surfaces such as submerged rocks along island or continental edges. These polyps reproduce and become a colony, which features a base attached to the hard surface, an edge zone where new coral polyps are produced, and an upper surface that is exposed to light.
Reef-building corals are found about 50 meters (165 feet) below sea level, in the photic zone, which is the depth to which sufficient sunlight penetrates the water, and photosynthesis occurs. Coral polyps have a symbiotic relationship (a prolonged association between two organisms of different species that may benefit each other) with zooxanthellae, a single-celled organism; these cells within the tissues of the polyps carry out photosynthesis, and produce nutrients that are used by the coral polyps (up to 90% of their nutrients). Because of the photosynthesis needed for this relationship, coral reefs grow faster in clear water, which admits more sunlight. Without the zooxanthellae, coral would grow too slowly to form reef structures.
There are a number of different forms of coral reefs. The three main types are fringing, barrier, and atoll. A fringing reef is directly attached to a shore, or borders it with a shallow lagoon or channel in between. A barrier reef is separated from an island shore or mainland shore by a deep lagoon. An atoll reef is a relatively circular or continuous reef which goes completely around a lagoon with no central island. This is often formed when a fringing reef forms around a volcanic island that retreats completely below sea level; the coral will continue to grow upward, forming the atoll. Other reef forms include a patch reef, which is an isolated, nearly circular reef, usually within a lagoon; an apron reef, which resembles a fringing reef, but is shorter and more sloped, and extends out and downward from a point or shore; a bank reef, which is a linear or semi-circle-shaped outline that is larger than a patch reef; a ribbon reef, which is long and narrow, and usually associated with an atoll lagoon; and a table reef, which is isolated and similar to an atoll reef, but has no lagoon.