Doctorfish, Acanthurus chirurgus

The Doctorfish (Acanthurus chirurgus), known also as the Doctorfish Tang, is a tropical marine fish that is common in the Atlantic Ocean.

Achieving a maximum length of 39 centimeters, this fish gets it common name for the structures called “scalpels”, which are found on either side of the caudal peduncle. The “scalpel” is utilized during fights with other doctorfish and as a defense mechanism against predators. Its coloration usually varies from blue-grey to dark brown. Ten to twelve vertical bars are always featured, but often faint. The edges of the caudal, anal, and dorsal 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, also, but it is neither a subspecies nor a regional mutation. It has only been documented a limited number of times.

The fish is usually found among rocky outcrops and coral reefs.

Its distribution includes the Atlantic from Massachusetts to Brazil, and the tropical west coast of Africa. It has the most widespread distribution of its genus of 38 species.

This fish spends its daylight hours grazing on algae and organic detritus. The teeth are specially shaped for scraping algae and plant matter from the rocks. Due to it swallowing 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.

Spawning takes place during the evening hours in a group event. Each egg is less than a millimeter in diameter and contains a small amount of oil for flotation. The translucent and plankton-like larvae hatch within 24 hours of fertilization. They are laterally compressed and diamond shaped with large eyes and pectoral fins. Many body parts, such as the scales and the dorsal and anal fins, don’t develop until the larvae have achieved 2 to 6 millimeters long. The “scalpel” doesn’t appear until they achieve about 13 millimeters long. As the “scalpel” grows, the anal and dorsal spines get smaller. Once the fish achieves around 25 millimeters long, it moves to the bottom where it continues to grow, eventually reaching sexual maturity in about nine months.

Image Caption: Acanthurus chirurgus. Credit: Brian Gratwicke/Wikipedia (CC BY 2.0)