Acanthuridae

Acanthurids are small in width in proportion to their length, and tend to be somewhat elliptical in shape. Ancathurids have a (sometimes toxic) blade-like scale that is set on the caudal peduncle(the narrowest part of the fish's body between the anal fin and the tail fin ), in the form of either stationary plates or spines that move with the muscles of the fish. Such spines are located in the fins of the fish and and gives it control to either fan out or fold down its fins. Unicornfish have the stationary plates on their caudal peduncle that protrude horizontally from the body. Surgeonfishes are known by the spine or spines protruding from both sides of the tail. The sharp spine(s) can be used as a defense against predators by slashing with its tail.