Dark brown with a pale band from the mouth and a crescent-shaped white margin on the tail.
To 30 cm. Dark brown on the upper body shading to a paler belly. Two marks identify it: a pale crescent across the tail, which is the name, and a bar running back from the eye to the base of the pectoral fin, which can be yellow or black depending on the fish. Juveniles are strongly dark above and white below.
Two others in the same genus are the confusion. The boomerang triggerfish has a curved boomerang mark behind the eye rather than a straight bar. The masked triggerfish is plainer and sandier and lives deeper. A third point matters for this site: in the Red Sea this species is replaced by the bluethroat triggerfish, so a halfmoon triggerfish recorded in the Red Sea would be a misidentification.
It lives on reef between the surface and about 80 m, feeds on bottom invertebrates, and is usually alone. When threatened it backs into a hole and locks its first dorsal spine upright so it cannot be pulled out. It is harmless.
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.