Patch reefs are aggregations of a few corals isolated from other reefs by sandy areas or seagrass beds. Fringing reefs closely follow shorelines sometimes with a narrow, shallow lagoon between the reef and land. Most Caribbean reefs are fringing reefs. Barrier reefs grow parallel to the coast, are often large and continuous, are separated from land by a wider lagoon, and are only likely to occur where there is a wide continental or insular shelf. The Great Barrier Reef in Australia is the largest barrier reef in the world.
Atolls are circular shaped reef crests surrounding a central lagoon.
In the Indo-Pacific, they develop during slow subsidence of volcanic islands and grow on the perimeter of the island, while in the Caribbean their origin is not understood as well. A coral may consist of a single polyp or a colony of thousands of polyps that are linked by a common gastrovascular system through which they share food, water, and wastes with surrounding polyps. Coral polyps have tubular, saclike bodies with a ring of tentacles surrounding their mouths. The non-tissue layer between the gastrodermis and the epidermis is called the mesoglea. Nematocysts are also found in the tentacle tips.