Coral reefs are submerged structures consisting of calcium carbonate secreted by corals. Coral reefs are colonies of small animals found in marine waters that enclose few nutrients. The majority of coral reefs are constructed from stony corals, which then consist of polyps that come together in groups. The polyps are like small sea anemones, to which they are very closely related. Unlike the sea anemones, coral polyps secrete hard carbonate exoskeletons which provide support and protections for their bodies. Reefs grow the best in shallow, clear, warm, sunny and agitated waters.
Frequently called “rainforests of the sea”, coral reefs create some of the most diverse ecosystems on Earth. They take up less than 0.1 percent of the world’s ocean surface, about half the entire area of France, yet they host 25 percent of all marine species.
The majority of the coral reefs were created after the last glacial period when melting ice caused rising sea levels and flooding of the continental shelves. This means that most coral reefs are less than 10,000 years of age.
There are three principal types of reef. Fringing reef is directly attached to a shore, or borders it with a prevailing shallow channel or lagoon. Barrier reed is a reef that is separated from a mainland or island shore by a deep channel or lagoon. Atoll reef a more or less circular or continuous barrier reef that extends all the way around a lagoon without a central island.
Coral reef ecosystems have distinct zones that are representation of different kinds of habitats. Normally, the three major zones are recognized: the fore reef, reef crest, and the back reef.
Image Caption: A Blue Starfish (Linckia laevigata) resting on hard Acropora coral. Credit: Richard Ling/Wikipedia (CC BY-SA 3.0)