The Dusky surgeonfish, from the Acanthuridae family is also known as the Brown surgeonfish, Blackspot surgeonfish, Lavender tang or Spot-cheeked surgeonfish. It is one of the most abundant fish that can be found on coral reefs. It is brown or bluish brown in colour, often with purple gradients. It has small orange spots on the head and chest. There is a black spot at the end of the dorsal and anal fin bases; a thin blue margin on the dorsal and anal fins; and a black margin around the caudal peduncle blade. Pelvic fins are brown. The groove of the caudal spine is encircled by a narrow black margin. The lips of this fish are blackish brown and its upper teeth tend to be pointed and used to crop algal turfs. This surgeonfish can grow up to 21 cm in length. Sexual dimorphism is not obvious but males display courtship colours. Aggression is displayed when upper back and dorsal fin lighten to a distinct yellow colour.
The Dusky surgeonfish is widely present across the Indo-Pacific region, from the Red Sea and East African coast to Pitcairn and Hawaiian Islands, north to southern parts of Japan and southwards to New South Wales, Australia. Despite its small size, this surgeonfish is aggressive. It is abundant on shallow coral reefs and rocky bottoms, as well as below the surge zone. It’s diet consists of filamentous algae; mostly red algae and this species is categorized as a grazer and herbivore browser. Adult individuals form small groups but are also seen in large group. Juveniles are seen in multiple species gatherings.
During the spawning season, adults move from feeding grounds to spawning sites that are located at the most seaward part of the reef or at channels between the lagoon and open ocean. They form spawning aggregations that consistently rise from the bottom up to 3metres height and then swim back to the substrate. Spawning season varies according to geographical location; it occurs during the full moon or new moon in November/December in Aldabra atoll while at the Great Barrier Reef it happens from February to April.
According to the IUCN Red List, this species is listed as ‘Least Concern’. Fishing and the aquarium trade are potential threats, but the species is common and dominant within most of its range. It is also present in Marine Protected Areas and marine reserves. Although fished, it is not considered threatened. There are no specific conservation programmes in place for this species.
Coral reef degradation due to anthropogenic activities may have long term impacts on surgeonfish as the majority of this species spend most of their life stages on the reef.
 ‘The IUCN Red List of Threatened Species’.