Coral reefs

Corals are animals related to sea anemones and jellyfish. Many reef-building species live in colonies of small animals called polyps. Stony coral polyps draw dissolved calcium from seawater and form it into calcium carbonate skeletons; the gradual growth and accumulation of these structures creates reef habitat. Reefs can fringe a coast, form a barrier separated from land by a lagoon, or develop as an atoll—a ring of reef around a lagoon, often associated with the subsidence of a volcanic island. Reef-building corals are common in clear, sunlit tropical and subtropical waters, while other coral communities also occur in cold, deep seas.

Many shallow-water reef-building corals host microscopic algae, often called zooxanthellae, within their tissues. The coral provides a protected place to live, while the algae use sunlight to photosynthesize and share energy-rich products with the coral. This mutualistic relationship helps explain why many familiar tropical reefs grow in clear, shallow water. It is not universal: deep-sea corals lack these algae and obtain food from plankton and other organic matter.

The structures created by corals provide food, shelter and breeding habitat for fish and many other organisms. Reefs are also connected to nearby ecosystems such as mangrove forests and seagrass beds. The United Nations Environment Programme reports that coral reefs cover less than one per cent of the ocean floor yet support at least a quarter of marine species. For people, reefs help sustain fisheries and tourism and can reduce the force of waves, storms and coastal erosion. Their importance is ecological, economic and cultural, especially for communities living near coasts and islands.

When corals are stressed, they may expel the algae living in their tissues. The loss of the algae and their pigments makes the coral appear white, a response known as bleaching. Bleaching is not itself proof that a coral has died: some corals recover if conditions improve, but stress can leave them more vulnerable to disease and death. Unusually warm water is a major cause of bleaching, though cold, excessive light, pollution and other stresses can also contribute. Reefs also face ocean acidification, sediment and nutrient pollution, habitat damage, disease and unsustainable fishing. These pressures can interact, reducing the ability of reefs to recover.

Coral conservation combines local measures—such as improving water quality, limiting destructive fishing and protecting important habitats—with action on global drivers, particularly greenhouse-gas emissions that warm and acidify the ocean. Scientists and conservation groups also monitor reef conditions and test restoration methods, including growing coral fragments in nurseries before transplanting them to damaged sites. Restoration can help particular places, but long-term reef health also depends on reducing the stresses that caused damage and allowing ecosystems time to recover.