What is a coral polyp?

A coral polyp is a small invertebrate animal, often just millimetres wide, related to sea anemones and jellyfish. Each polyp has a sac-like body topped with a ring of stinging tentacles used to capture tiny prey. Reef-building polyps extract calcium and carbonate from seawater to construct a hard limestone skeleton beneath them, and as generations of polyps live and die, these skeletons accumulate into massive reef structures. Most reef corals also host tiny algae called zooxanthellae inside their tissues, which photosynthesise and supply the polyp with much of its food, giving corals their vivid colours. When stressed by warm water, corals expel these algae and turn white, a process called bleaching.