What type of cnidarian makes up an underwater reef?

A coral is the cnidarian that makes up an underwater reef. More precisely, reef-building corals are colonial animals in the cnidarian phylum, especially hard, or stony, corals in the class Anthozoa.

Each coral colony consists of many tiny animals called polyps. These polyps secrete calcium carbonate, building hard skeletons beneath and around themselves. Over many generations, the accumulated skeletons form the massive structures known as coral reefs. The living coral occupies only the reef’s growing surface; much of the underlying reef is the remains of earlier colonies.

Corals are often mistaken for plants because they stay attached to one place, but they are animals related to sea anemones and jellyfish. Many shallow-water corals also live with microscopic algae called zooxanthellae, which use sunlight to produce food for themselves and their coral hosts. This partnership helps explain why many tropical reefs need clear, warm, shallow water.

Coral reefs are not made by every cnidarian. Jellyfish and most sea anemones do not build reef skeletons, although some other cnidarians, such as fire corals, can contribute to reef habitats. Coral reefs support exceptionally diverse marine communities and are sometimes called the rainforests of the sea.