A clear-water outer-reef surgeonfish, and easily confused with the powderbrown of the same reefs.
To about 21 cm, dark brown to near black over the body with a small white patch below and just behind the eye, a yellow band along the base of the dorsal fin and another along the base of the anal fin, a white bar at the tail base and a yellow patch surrounding the caudal blade. The confusion is with the powderbrown surgeonfish Acanthurus japonicus, which shares the yellow fin bands but carries white across the whole front of the face rather than a small patch under the eye, and has an orange stripe along the outer dorsal fin. A. japonicus is the common one in the Philippines and northern Sulawesi; A. nigricans is the one to expect on clear oceanic reef.
It grazes filamentous algae from hard substrate in the shallow, well-lit, surge-washed part of the reef, singly or in loose groups, and it moves fast between grazing patches. It favours exposed outer reef with clear water and strong water movement, which is why it is a routine fish in Papua New Guinea and on the exposed Bali slopes but genuinely uncommon in the sheltered, silty conditions of Lembeh Strait and the Dauin coast. Where it does turn up on those coasts it is on the few current-swept coral points rather than on the sand.
It does not tolerate a close approach for long and tends to circle back past the same patch of reef, so waiting on a grazing area is more productive than following. Recorded from about 2 to 30 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.