Coral is a cnidarian animal that can build the limestone framework of a reef.
Reef-building corals are colonies of tiny polyps. Each polyp secretes a cup-like skeleton of calcium carbonate, and repeated growth over many generations can create large underwater structures. The living tissue may contain symbiotic dinoflagellate algae, commonly called zooxanthellae, which provide energy through photosynthesis.
Corals are related to sea anemones and jellyfish, not to plants or rocks. Their polyps possess stinging cells called cnidocytes, a defining feature of the phylum Cnidaria. Some corals live without photosynthetic partners and occupy deeper or darker habitats.
Warm-water reefs are highly diverse ecosystems, but unusually warm seawater can cause bleaching by disrupting the coral-algae relationship. Bleached corals are stressed, not automatically dead, and may recover if conditions improve. Ocean warming, acidification, pollution and destructive fishing all threaten reef systems.