damselfish, common name for members of the large family Pomacentridae, marine fishes of tropical waters. Common in the West Indies and along the Florida coasts are the sergeant-major, named for its vertical stripes, and the reef fish, found among coral reefs. The clownfish and blue devil are popular aquarium species. Males of this family guard the eggs zealously. Damselfishes rarely grow to more than 6 in. (15 cm) in length. Certain damselfishes are found associated with sea anemones, which are injurious to the fishes' predators, and which afford protection to the damselfish. Damselfish are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Perciformes, family Pomacentridae.