A tall Indian Ocean tang with fine vertical lines and a huge dorsal fin raised in display.
To 40 cm, commonly 32. The body is deep and disc-like, and the dorsal and anal fins are so large that raising them roughly doubles the apparent depth of the fish, which is where the name comes from. The body carries alternating dark and pale vertical bands, the face is covered in fine white or yellow spots, the rear body and fins are spotted and streaked, and the tail is usually yellowish and spotted. Juveniles look different: bolder, higher-contrast banding with proportionally even larger fins, and they are alone where adults are usually in pairs.
The Pacific sailfin tang was long treated as the same species and is now separated again. Geography is the reliable check: that one is Pacific, this one is Indian Ocean and the Red Sea, and they do not meet in the Maldives.
It grazes filamentous algae on lagoon and seaward reef between 3 and 30 m, and deeper on seaward slopes. Juveniles are usually among staghorn coral in lagoons. Like all surgeonfish it carries a hinged blade on each side of the tail base that folds out when the fish is threatened; it is sharp, and the risk is entirely from handling.
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.