Coral Reefs are marine structures composed primarily of calcium carbonate. Stony Corals belonging to the order Scleractinia, are the primary source of calcium carbonate in a reef system. The calcium carbonate is secreted by coral polyps to form a rigid exoskeleton called a calyx. The calyx is composed of the basal plate that the polyp sits on, and the theca that surrounds the polyp. Periodically the polyp will lift off of its base and secrete a new basal plate, thus creating a small chamber below the new basal plate and elevating the coral. Through this action some corals can grow 10-20 cm a year. When an individual polyp dies it is replaced, and the colony grows indefinitely.

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Due to a symbiotic relationship with a photosynthetic algae known as zooanthellae, sunlight is the primary factor for good coral habitat. This single-celled organism produces food for itself through photosynthesis. It gives some of this food to the coral, and the coral provides protection and access to light for the algae. This relationship allows the corals to grow into reefs. Reef-building corals are usually found in clear water and depths of 70m or less. Because sunlight is so paramount, shallow-water corals only occur at latitudes between 30 N and 30 S. Warm water between the temperatures of 23 and 29 C is another necessity for healthy coral growth. For this reason corals rarely form in areas affected by strong coastal upwelling, such as along the west coast of North and South America.

(marinebio)