Coral reefs represent one of the most diverse and productive ecosystems in the world. They are found in shallow, clear water environments, and require tropical and subtropical warm water temperatures. Therefore, they are often distributed in a band between 30 degrees north and 30 degrees south of the equator. A coral community is formed by a wide diversity of small sessile animals of the phylum Cnidaria, which includes the coral themselves, hydras, jellyfish, and sea anemones. Corals live in colonies, which are formed by many individuals each called a polyp. These polyps continuously secrete calcium carbonate, which serves as substrate for the colony. These colonies build up slowly over time to form eventually large and complex coral reefs. The growth rate of a coral reef structure is very variable depending on the species of coral and environmental conditions, ranging from 0.3 to 10 centimeters per year. Different coral species build structures that vary in size and shape (i.e., brain corals, fan corals, etc.), which increase the diversity and complexity of the coral reef habitat. Corals have a symbiotic (mutually beneficial) relationship with algae called zooxanthellae. This algae performs photosynthesis providing food for the coral, and in exchange the coral provides them with protection and access to light.
Coral reefs are generally classified in three types. Fringing reefs, the most common type, which projects seaward directly from the shore of islands or continents. Barrier reefs, which are platforms separated from the adjacent shore by a bay or lagoon. And atolls, which are found in the top of submerged volcanoes. These are usually circular or oval with a central lagoon.
In addition to coastal protection from erosion, coral reefs provide habitat, food, and shelter to a large variety of organisms including sponges; mollusks such as sea slugs, oysters, and clams; crustaceans like crabs and shrimp; sea worms; echinoderms like star fish and sea urchins; fungi; sea turtles; and many species of fish. Coral reefs also support important commercial fisheries and serve as major tourist attractions.
