Coral reefs are the largest natural structures in the world. They are the result of a remarkable relationship between coral animals, known as polyps, and microscopic algae (zooxanthellae) living in their tissues. The polyp, resembling a small sea anemone, is able to feed itself using stinging cells found on its tentacles which paralyze passing plankton. The plankton is digested but supplies only a small part of the nutritional requirements of the polyp. The remainder comes from the zooxanthellae which convert sunlight, carbon dioxide and their own wastes into oxygen and carbohydrates. These carbohydrates are also used by the polyp to make calcium carbonate in a process known as calcification. This material forms the skeleton of the coral and eventually the framework of what we recognize today as a coral reef.
