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Coral Polyps
Corals are members of the animal phylum Cnidaria, class Anthozoa, which consists of more than 6,000 species of organisms, including anemones and sea fans. An individual coral organism is called a polyp, which in zoology, is a type of animal form, characteristic of anthozoans. Coral polyps are elongated, sac-like animals with tentacles fringed around the mouth opening. They come in a wide array of colors, grow up to 1/8 inch in height, and sit inside a cup-shaped skeleton called a calyx. These skeletons are created by the polyp through secretions of calcium-carbonate or CaCO3.
All coral polyps secrete CaCO3, but not all corals build reefs. Reefs are created by colonies of coral, and some species, such as Fungia sp., are solitary organisms. Also, some species do not produce adequate amounts of CaCO3 for reef building. The types of coral that build reefs are called hermatypic or "stony coral."
Building the Reef
Coral reefs begin to develop when stony coral polyps attach to portions of land forms that are submerged underwater. What is referred to as the reef's limestone --- which is a form of CaCO3, is actually the same CaCO3 that makes up the skeleton of stony coral. So for all practical purposes, a coral reef is an enormous formation of skeletons.
The appearance of the coral reef depends on the type of coral producing it. Each species and colony of coral has distinct characteristics that are reflected in the reefs they build. Some of the basic reef formations include "digitate coral" that has finger-like extensions or branches; "table coral" with branches that are constructed in a table-like formation; "elkhorn coral" that is comprised of flat branches; "submassive coral" that has wedges or knobs extended from its base; "mushroom coral" that have the appearance of mushroom caps; and "massive coral" that consists of ball and boulder shaped structures
Types of Reefs
Regardless of the appearance of the reef, coral reefs grow and expand into one of three main structural types: barrier, atoll or fringing.
Barrier reefs frame shorelines at a great distance and are separated from their neighboring land mass by a small body of open water such as a lagoon. Atolls are generally circular, with a lagoon situated in the middle of the reef. Fringing reefs extend toward the sea directly from the shore, and form fringes or borders along the shoreline.
Coral Ecosystems
An ecosystem is a network of organisms and non-living things that are interdependent of each other within a specific environment. Coral reefs play a crucial role in aquatic ecosystems by providing a habitat for more than 4,000 species of various marine animals. Experts report that at least 25 percent of marine organisms -- such as fish, crustaceans, and anemones -- thrive among the coral reefs of the world.