Acanthurus tristis belongs to the order of Perciformes, from the family of Acanthuridae. Common names include Indian Ocean mimic surgeonfish, Indian Ocean mimic tang, Black-cheek surgeonfish and Yellow-spot surgeonfish. The caudal fin has a narrow white margin. There are orange patches behind the gill opening that extend ventrally behind the base of the pectoral fins. It has a total of 8 dorsal spines; 233 dorsal soft rays; 3 anal spines and 22 to 29 anal soft rays. It can reach a maximum length of 25cm. Surgeonfish do not exhibit prominent sexual dimorphism. However, males display courtship colours. Juveniles mimic Centropyge eibli. The latter is pale greyish in colour with vertical streaks running across the sides. Its eye is rimmed with orange and its tail and tail base are black with a blue margin at the rear end of the caudal fin.
 ‘Acanthurus Tristis Summary Page’.
Acanthurus tristis is present in the Indian Ocean, from the Bay of Bengal and the west of Andaman Sea to Maldives and Chagos Archipelago, and east side of the islands of southern Indonesia to Bali. This species inhabits shallow lagoons and seaward reefs in regions of mixed coral, rock or sand. It can also be found in deeper waters, usually over sand and rubble bottoms. Depth range is between 2 to 30m. It is a detritivore, that is, it feeds on dead matter.
According to the IUCN Red List, this species is listed under the ‘Least Concern’ category. It is found in the Eastern Indian Ocean region. In certain areas it is commonly present while it is rare in other parts of its range. No species specific conservation measures are in place but it is present in many MPAs (Marine Protected Areas). Aquarium trade and collect are potential threats.
In Western Thailand, this species is a targeted food fish.