sea fan, colonial marine animal forming erect, flattened, branching colonies in tropical and subtropical waters. Colonies may be several feet high and are often colorful, with purples, reds, and yellows predominating. The individuals, or polyps (see polyp and medusa), have eight feathery tentacles and feed on plankton organisms. Sea fans are classified in the phylum Cnidaria, class Anthozoa, order Gorgonacea.