Acanthurus achilles

Acanthurus achilles, the Achilles tang, redtail surgeonfish or redspot surgeonfish, is a marine ray-finned fish belonging to the family Acanthuridae, the surgeonfishes, unicornfishes and tangs. This fish is found in the Pacific Ocean.

Taxonomy
Acanthurus achilles was first formally described in 1803 by the English biologist George Shaw but Shaw did not give a type locality and no type specimen is known. The genus Acanthurus is one of two genera in the tribe Acanthurini which is one of three tribes in the subfamily Acanthurinae which is one of two subfamilies in the family Acanthuridae. This species is known to hybridise with Acanthurus nigricans.

Etymology
Acanthurus achilles has the specific name achilles which Shaw did not explain. It is thought that the name refers to Achilles, the mythical hero of the Trojan War, and may be an allusion to the sharp bony plates on the caudal peduncle.

Description
Acanthurus achilles has an elongated oval-shaped body with a steep dorsal profile to the head. The small, protrusible mouth is positioned low on the head with between 8 and 28 teeth fixed on each jaw, each having a flattened, serrated tip. The dorsal fin is supported by 9 spines and between 29 and 33 soft rays while the anal fin contains 3 spines and 26 to 29 soft rays. There is a single mobile spine on each side of the caudal peduncle which folds down into a slit. The caudal fin is concave. The overall colour is bluish-black with an irregular oval orange blotch on the rear of the body, a white bar on the gill cover, a wide orange bar on the caudal fin which also has a white posterior margin. There is a pale blue ring around just inside the snout tip. This species reaches a maximum total length of 24 cm.

Distribution and habitat
Acanthurus achilles is found in various reefs of Oceania, up to the islands of Hawaii and Pitcairn. The fish is also, although less commonly, found in the Mariana Islands and even some reefs in southern Mexico and Guatemala] It is a benthopelagic species which inhabits the clear waters on the seaward side of reefs.

Biology
Acanthurus achilles is herbivorous, grazing largely on benthic algae. They are territorial and monogamous.