Coral reefs
Coral reefs (also known as Sea Gardens) grow in tropical seas in the Sunlit Zone/Layer, where there is mild wave action, not so strong it tears the reef apart yet strong enough to stir the water and deliver sufficient food and oxygen. Coral reefs also need nutrient-poor, clear, warm, shallow water to grow. Coral is built up from millions of skeletons from tiny animals called polyps, which are related to sea anemones.

Coral reef biology
The building blocks of coral reefs are the "skeletons" of generations of reef-building algae, corals, and other organisms that are composed of calcium carbonate. For example, as a coral head grows, it lays down a skeletal structure encasing each new polyp. Waves, grazing fishes (such as parrotfish), sea urchins, sponges, and other forces and organisms break down the coral skeletons into fragments that settle into spaces in the reef structure. Many other organisms living in the reef community contribute their skeletal calcium carbonate in the same manner.

Coralline algae are actually the main contributors to the structure, at least in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae contribute to reef-building by depositing limestone in sheets over the surface of the reef and thereby contributing to the structural integrity of the reef. Many coralline algae species form nodules, or develop on the surface of fragments, enlarging these. The crust-forming species protect coral reefs by withstanding and mitigating wave pressures that would destroy most corals. This crust often forms a protective ridge on the outer edge of a reef (reef crest or reef margin), particularly in the Pacific (Castro and Huber, 2000; Nybakken, 1997).

Coral reef formations
Coral reefs can take a variety of forms, defined as the following;

Apron reef — short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
Fringing reef — reef that is directly attached to shore or borders it with an intervening shallow channel or lagoon.
Barrier reef — reef separated from a mainland or island shore by a deep lagoon; see Great Barrier Reef.
Patch reef — an isolated, often circular reef, usually within a lagoon or embayment.
Ribbon reef — long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
Table reef — isolated reef, approaching an atoll type, but without a lagoon.
Atoll reef — a more or less circular or continuous barrier reef surrounding a lagoon without a central island; see atoll.