Coral reef A fringing reef forms around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef. Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide. Where the level of the underlying earth allows, the corals grow around the coast
Florida Reef open ocean. Channels between the Keys allow brackish water from the bays to flow onto the reefs (especially in the middle Keys), limiting their growth. The Florida Reef consists of two ridges separated from the Florida Keys by the Hawk Channel. Closest to the Keys is a sand ridge called "White Bank", covered by large beds of sea grass, with patch reefs scattered across it. Further out to sea on the edge of the Florida Straits is the second ridge forming the outer reefs, covered by reefs and hard banks composed of coral rubble and sand. Almost 1,400 species of