Body thin, oval in shape, with relatively long dorsal and anal fins and a crescent tail. Uniform in color without body bars. Can change from bluish gray to dark brown, and can pale or darken dramatically. Markings radiate from the eye. Dorsal, anal and tail fins with a blue or white border. Base of tail with a sharp spine (like a surgeon's scalpel), may have a pale band behind the spine (A. bahianus).
Size up to 38 cm.
Inhabits shallow bottoms with coral or rocky formations, down to 40 m. Usually occurs in groups of five or more individuals. Mainly a diurnal species. Feeds on algae. The sharp spines on both sides of the base tail are used as defensive weapons by slashing their tails from side to side. When not in use, they are folded backwards against the body.
Common to occasional Florida, Bahamas and Caribbean.