Deep blue as an adult and bright yellow as a juvenile. Adults form roving shoals that cross damselfish territories in numbers too great to be driven off.
To about 35 cm, deep blue as an adult with fine darker lines, and bright yellow as a juvenile, which is often taken for another species. The blade at the tail base is white and clearly visible against the blue.
It is a grazer and it moves in roving shoals, sometimes of a hundred or more. The shoal is the point: a single blue tang is driven off a damselfish’s algal patch, and a shoal is not, so the group is a way of getting at food that one fish cannot reach. It also means the fish that has been alone all morning is suddenly in a crowd crossing the reef at speed. The blade is a cutting edge and it is used against other fish. It cuts a hand that holds one, and that is the whole of its hazard.
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.