Coral reefs are arguably the world's most beautiful habitats. Coral reefs have been called the rainforests of the oceans, because of the rich diversity of life they support. Scientists have not yet finished counting the thousands of different species of plants and animals that use or live in the coral reef. They are massive underwater structures formed by the limestone skeletons of tiny invertebrate animals. Reefs house a greater diversity of body forms, chemistry, and animal phyla. Coral animals begin life as free-floating larvae, but settle on the sea floor in sedentary colonies. The term "coral" applies both to these animals and to their skeletons, particularly the skeletons of stone-like corals (Discover 1997). .
Many different organisms, including mollusks, sponges, and worms, help shape reefs, but hard corals and various algae are the major architects. In effect, the corals build limestone, because their skeletons are made of Calcium Carbonate. The skeletons deposited by these corals and other organisms accumulate, along with sand and other debris, to form the backbone of the reef. Over tens of thousands of years, chemical and mechanical changes turn the reef into true rock.
The body of a coral animal consists of a polyp, which is the living portion of the coral. A polyp is a hollow, cylindrical structure attached at one end to a surface, the other end is a mouth surrounded by tentacles, which gather food and can sting prey to paralyze it. Microscopic algae live within the animal tissues in a symbiotic relationship. The algae turn sunlight into carbon and sugars, which are then available to the polyp. In turn the polyp filters particles out of the water and excretes waste (nitrogen and phosphorus) that becomes available to the symbiotic algae. It's this very tight nutrient recycling within the coral itself that allows these corals to live in very low nutrient waters.
There are three kinds of reefs: atolls, barrier reefs, and fringing reefs.