Sea anemones ( ə-NEM-ə-nee) are a group of predatory marine invertebrate animals constituting the order Actiniaria. Because of their colourful appearance, they are named after the Anemone, a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. As cnidarians, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra. Unlike jellyfish, sea anemones do not have a medusa stage in their life cycle.

A typical sea anemone is a single polyp attached to a hard surface by its base, but some species live in soft sediment, and a few float near the surface of the water. The polyp has a columnar trunk topped by an oral disc with a ring of tentacles and a central mouth. The tentacles can be retracted inside the body cavity or expanded to catch passing prey. They are armed with cnidocytes (stinging cells). In many species, additional nourishment comes from a symbiotic relationship with single-celled dinoflagellates, with zooxanthellae, or with green algae, zoochlorellae, that live within the cells. Some species of sea anemone live in association with clownfish, hermit crabs, small fish, or other animals to their mutual benefit.

Sea anemones breed by liberating sperm and eggs through the mouth into the sea. The resulting fertilized eggs develop into planula larvae which, after being planktonic for a while, settle on the seabed and develop directly into juvenile polyps. Sea anemones also breed asexually, by breaking in half or into smaller pieces which regenerate into polyps. Sea anemones are sometimes kept in reef aquariums; the global trade in marine ornamentals for this purpose is expanding and threatens sea anemone populations in some localities, as the trade depends on collection from the wild.

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