CORAL AND CORAL REEFS

Coral reefs are extraordinary geographic, geologic formations. Geographic, because they are found in seas within 20 degrees, mostly, of the equator. Geologic, because their colonial skeletons form structures, usually adjacent to land. The form of the reef, and its relationship to land (or the absence of it) tells us about the ancient history of the area. These two aspects determine the classification of coral reefs. Fringing reefs form directly from the shore. They grow along the shore, maintaining particular depth and habitat. If the land sinks, or is eroded so that the shoreline moves away from the reef (in an inland direction) a barrier reef results. An island that completely disappears leaves a ring, perhaps incomplete, called an atoll. Table reefs form on the tops of seamounts; they lack both adjacent land and an atoll in the middle. Patch reefs are found in lagoons; they are separate from the atoll and are usually surrounded by sand.

Coral reefs are built, rather than deposited. The strata of the white cliffs of Dover, England, are the result of many tiny animals dying whose skeletons simply accumulated as they fell to the bottom. Coral reefs are composed of calcium carbonate or limestone. Each shell and each layer of the colony was constructed and extended by the animals themselves.

Each tiny coral is the result of a remarkable relationship between a coelenterate related to jellyfish and sea anemones and one celled algae, called zooxanthellae. The algae burrow into the tissue of the coral in great numbers, providing the color of living coral reefs, mainly yellow, brown, green and pink. Coral skeletons are usually white but can be red, blue or the now rare black coral, valued for jewelry. The interaction between the coral animal and the alga is complex, and not yet well understood. The coral supplies carbon dioxide, food, shelter and support for the plant, which returns oxygen, food, and reef-building energy for the production of the calcium carbonate shells that protect them both and form the colony. Without the zooxanthellae we might still have colonies of corals, similar to jellyfish, but we would not have reefs.

Coral is believed to have first evolved in the area of the Malay Peninsula. As aresult of this and the diversity of environments found here, there is a great variety of corals, particularly near Phuket. 75 genera containing 200 species are found in this area alone. Typically, a coral reef is composed of many kinds of corals. Both the structure and the variety attract the broadest spectrum of animals found in any environment. 33 phyla, representing nearly every major group of animals, from sponges to the chordates (including vertebrates such a fish, reptiles and mammals) live in, on, or next to coral reefs. Visiting a coral reef as a diver or snorkeler allows you to watch and understand this colorful and complex community.