The Tiny Animals That Build Reefs

A coral reef can look like an underwater garden. Some corals resemble branches; others look like rounded rocks. However, the living builders are animals called polyps. A polyp has a small body and tentacles around its mouth.

Many corals live in colonies, or groups, containing large numbers of polyps. Stony corals build hard skeletons from a material called calcium carbonate. As colonies grow over many years, their skeletons help form a reef. The structure a diver sees can be much larger than any single animal building it.

Many reef-building corals have tiny algae living inside their bodies. The algae use sunlight to make food and share some of it with the coral. In return, the coral provides a protected place to live. This partnership helps explain why sunlight matters to many shallow-water reefs.

Stress, such as unusually warm water, can cause a coral to lose these algae and turn pale or white. This is called bleaching. A bleached coral may still be alive, but it has lost an important source of food. It can survive, although continued stress increases the risk of death.