A blue-lined mid-water unicornfish that changes colour within seconds.
To about 60 cm, deep-bodied and grey-brown to blue-grey, marked with fine vertical blue lines on the flanks, blue spots on the head, blue lips and blue bands around the eye. Adults develop a bulbous snout rather than the true horn of some relatives. The blue markings can be intensified or switched off in a second or two, and a displaying fish looks like a different animal from a feeding one.
It feeds on zooplankton in open water off slopes and drop-offs, usually in loose aggregations that hang a few metres out from the reef, and returns to the reef to be cleaned. Because it is in mid-water rather than on the reef it is often passed by; the reliable approach is to stop at the drop-off edge and let the group settle. Recorded from about 5 to 60 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.