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Not all the limestone produced is created by corals, either. In some areas, particularly where there is very strong wave action, calcareous algaes are the primary producers of carbonate, forming algal ridges at the outer edge of the reef.

Coral Reef Architecture

Coral reefs are generally defined as falling into three main types: fringing reefs, barrier reefs and atolls. In a sense, these types also form a historical progression. As a fringing reef grows outward, a boat channel forms behind. As the reef widens, the boat channel becomes a lagoon, and the fringing reef graduates to a barrier reef. If the fringing reef began around an island, and the island subsequently sinks or the sea level rises, the final result is an atoll, a near circular reef surrounding a central lagoon.

Zooxanthellae and Corals

Reef-building corals have evolved an indispensable, symbiotic relationship with a type of yellow-brown algae called zooxanthellae, which is "farmed" in the tissues of the coral polyp. The relationship is mutually beneficial: the coral receives oxygen and nutrients, and the algae receives carbon dioxide and "fertilizer" in the form of animal waste.