any of about 250 species of small, primarily tropical marine fishes of the family Pomacentridae (order Perciformes) found in the Atlantic and Indo-Pacific oceans. Damselfishes are deep-bodied and usually have forked tails. They resemble the related cichlids, and, like them, have a single nostril on each side of the head. Many species are brilliantly coloured, often in shades of red, orange, yellow, or blue; most do not exceed a length of about 15 cm (6 inches)