Nature · Earth Science
The rainforests of the sea
A coral reef is a living underwater structure built by colonies of tiny animals called coral polyps. Though they cover less than one percent of the ocean floor, reefs shelter about a quarter of all marine species, making them the most biodiverse habitats in the sea.
Coral polyps secrete hard calcium-carbonate skeletons that cement together and build up over centuries into vast reefs. They rely on a partnership with tiny algae living in their tissues, which need clear, warm, sunlit water — which is why reefs thrive in shallow tropical seas.
From the shallow reef flat down the reef crest and steep fore-reef slope into the deep — each zone home to different life.
Reefs hugging the shore
Fringing reefs grow outward straight from the shoreline, forming a shallow platform of coral just offshore — the reef most snorkellers first meet.
Great walls parallel to the coast
Barrier reefs run parallel to the coast but far out, separated from land by a wide, deep lagoon. The Great Barrier Reef is so vast it is visible from space.
Coral rings around a lagoon
As a volcanic island slowly sinks, its fringing reef keeps growing upward, eventually leaving a ring of coral encircling a lagoon where the island once stood.
Isolated coral gardens
Patch reefs are compact, standalone mounds of coral rising from the seabed within a lagoon or shelf, each a self-contained oasis of marine life.