Bloch & Schneider, 1801
Body thin, oval in shape, with relatively long dorsal and anal fins and a crescent tail. Body blue overall, but can change from powder blue to deep purple, and pale or darken (A. coeruleus adult). Dorsal and anal fins edged in bright blue.
Base of tail with a sharp spine (like a surgeon's scalpel), white or yellow in color.
Juveniles bright yellow overall (A. coeruleus juvenile), intermediate with a blue head and body, but with a yellow tail. Changes from juvenile to intermediate and adult color phase are independent of the size, and sometimes a yellow phase juvenile may be larger than a blue-phased adult.
Size up to 39 cm.
Inhabits coral reefs, inshore grassy or rocky areas, down to 40 m. Can be solitary, but more often in large aggregations grazing on algae. Mainly diurnal. 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.
Abundant to common Florida, Bahamas and Caribbean.
Body more circular than in the Ocean surgeonfish (Acanthurus bahianus) and Doctorfish (Acanthurus chirurgus).