Coral reef

Corals are animals related to sea anemones and jellyfish. Reef-building stony corals live in colonies of small animals called polyps. Each polyp can take dissolved calcium from seawater and deposit calcium carbonate as a hard skeleton. As colonies grow and successive layers accumulate, these mineral structures form the framework of a reef. The visible living tissue is only a thin layer over much older skeleton beneath it. Not every coral builds a reef: some coral groups have soft bodies or make little contribution to massive reef structures.

Many shallow-water reef-building corals host microscopic photosynthetic algae, often called zooxanthellae, within their tissues. The coral provides shelter and materials the algae need; in return, the algae make food that supports the coral. This partnership helps explain why most tropical reef-building corals flourish in clear, sunlit water. Coral communities are not restricted to shallow tropical seas, however: other corals occur in cold, deep water, where deep-sea species obtain food in different ways and do not depend on these photosynthetic algae.

The branching and creviced reef structure creates feeding, shelter and breeding places for fish and many other organisms. Reefs are among the most diverse marine ecosystems and are connected to nearby habitats such as mangroves and seagrass beds. People also benefit from reefs through fisheries, recreation, tourism and natural protection of some coastlines from waves and storms. These ecological and social roles make the condition of reefs important well beyond the places where corals grow.

When stressed, a coral may lose or expel its symbiotic algae, causing its tissue to look pale or white. This is coral bleaching; a bleached coral is not necessarily dead, but it is stressed and may face a greater risk of disease or mortality if poor conditions persist. Unusually warm water is a major cause of mass bleaching, while pollution, sediment, damaging fishing practices and other local pressures can also harm reefs. Ocean acidification adds another challenge for organisms that build calcium-carbonate structures. The Global Coral Reef Monitoring Network's 2025 assessment identifies climate-driven marine heatwaves and bleaching as the primary driver of the global decline in hard-coral cover it reports, while emphasizing that trends differ among regions.

Reef management combines local measures—such as limiting pollution and destructive fishing and protecting key habitats—with action on the larger pressures caused by climate change. Monitoring helps scientists distinguish regional changes, identify disturbances and assess recovery. Networks such as the Global Coral Reef Monitoring Network assemble observations from many countries to inform conservation and management. Because reefs respond to both local conditions and global ocean change, protection depends on sustained observation and efforts at multiple scales.