Coral reefs are among the most diverse and productive communities on Earth. They are found in the warm, clear, shallow waters of tropical oceans worldwide. Reefs have functions ranging from providing food and shelter to fish and invertebrates, to protecting the shore from erosion. Although many corals resemble plants, they are actually members of the animal phylum Cnidaria. Most corals are colonial, which means that each coral is made up of many individual polyps connected by living tissue (the coenosarc). Each polyp has a cup-like shape with a ring of tentacles around a central opening (pharynx) that functions as both mouth and anus. The tentacles are tipped with stinging cells called nematocysts. Corals use the nematocysts to defend themselves and to capture prey. The body wall consists of three cell layers: the outer or ectoderm, the middle or mesoderm, and the inner or endoderm. There is no skeleton inside the polyp itself. Instead, the polyps sit on top of an external skeleton that is made from the polyp's secretions. One of the most interesting findings about coral are some of their reproducing habits. Horn coral, for example, depend on waves to break off pieces and carry them to new locations where the broken pieces start new colonies. The more famous coral forms huge deposits that take on the shape of small, underwater mountains of calcium carbonate. Corals are benthic organisms in the fact that they are stationary for the most part, and do not swim or drift in the ocean. All coral feed on plankton. Soft coral are filter feeders, filtering out plankton as the current passes through the porous structure of the coral. Hard coral have tiny "critters" located inside a limestone shell that rely on plankton that float by as their food source. Since they are very sensitive, coral require a very specific environment in order to survive. They are found generally in warm, shallow areas of the tropical oceans. Although t...
