Doctorfish

The doctorfish tang reaches a maximum size of 39 centimetres (15 in) and 5.1 kilograms (11 lb). Acanthurus chirurgus gets its common name for the structures called "scalpels", which are found on either side of the caudal peduncle. The "scalpel" is used during fights with other doctorfish and as a defense mechanism against predators. Its coloration generally varies from blue-gray to dark brown. 10 to 12 vertical bars are always present, but often faint. The edges of the caudal, dorsal, and anal fins are blue. There is also a faint blue ring that can be seen encircling the "scalpel" on either side.

There is a black morph, as well, but it is neither a subspecies nor a regional mutation. It has only been documented a limited number of times.

This species is typically found among rocky outcrops and coral reefs. Its distribution includes the Atlantic from Massachusetts to Brazil, and the tropical west coast of Africa, although the African records may be misidentifications of Acanthurus monroviae. It has been recorded twice recently (2012, 2016) in the central Mediterranean Sea.

Acanthurus chirurgus spends its daylight hours grazing on algae and organic detritus. Its teeth are specially shaped for scraping algae and plant matter from rocks. Because it swallows its food whole, it has a gizzard-like organ in the intestine filled with particles of sand which help to grind food before it starts the digestive process.