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any of about 24 species of marine fishes of the family Polynemidae (order Perciformes), widely distributed along warm seashores, often over sand. Threadfins have two well-separated dorsal fins and a forked tail, and are usually silvery in colour. Their name refers to their pectoral fins, each of which is divided into an upper, normally shaped fin and a lower section consisting of four to seven long, threadlike rays that serve as sensory devices; they are especially long in the young. Most threadfins are 30-60 centimetres (1-2 feet) long; one species, Eleutheronema tetradactylum of India, is about 1.8 metres (6 feet) long.