Corals belong to a group of cnidarians known as the Class Anthozoa. There are many types of coral and it should be noted that the term coral does not correspond to a single taxonomic class. Some groups of corals include:
- Order Alcyonacea (soft corals)
- Order Antipatharia (black corals and thorny corals)
- Order Scleractinia (stony corals)
Stony corals make up the largest group of organisms within the Class Anthozoa. Stony corals produce a skeleton of calcium carbonate crystals which they secrete from the epidermis of the lower part of their stalk and basal disc. The calcium carbonate they secrete forms a cup (or calyx) in which the coral polyp sits. The polyp can retract into the cup for protection. Stony corals are the key contributors to coral reef formation and as such provide the main source of calcium carbonate for the construction of the reef.
Soft corals do not produce calcium carbonate skeletons like those of stony corals. Instead, the contain tiny cacareous spicules and grow in mounds or mushroom shapes. Black corals are plant-like colonies that form around an axial skeleton that has black thorny structure. Black corals are found primarily in deep. tropical waters.
