butterfly fish, common name for certain members of the Chaetodontidae, a family of reef-dwelling tropical fishes that also includes the angelfishes and is closely allied to the spadefishes and the tangs. All have compressed bodies and small mouths and teeth. Butterfly fish are carnivorous, feeding on crabs, barnacles, and other invertebrates. The fast and aggressive common butterfly fish, 5 to 8 in. (12.5–20 cm) long, is marked by dark lines through the eyes and near the tail. The angelfishes have spines on their gill covers and long filaments on their dorsal fins. The queen angelfish, a good food fish that reaches 2 ft (60 cm) in length, is colored in blues and yellows; the smaller, more numerous common angelfish is similar. The French angelfish is black with yellow scale edgings; the black angelfish is solid black; and the bizarre rock beauty has a black body with yellow head, fins, and tail. The spadefishes are larger (up to 3 ft/90 cm) and faster than the angelfishes and are valued both as food and as game fishes. They are barred in black and white. The tangs have variable coloration. They include the violet-brown doctorfish or surgeonfish, the 8-in. (20-cm) blue tang, and the larger and more abundant ocean tang of deep waters. The butterfly fishes are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Perciformes, family Chaetodontidae.
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