A boxfish with a honeycomb pattern over the rigid shell and a pair of horns above the eyes. It changes colour markedly and slowly.
To 50 cm, commonly 25. The pattern is the name: a bold network of hexagons outlined in narrow dark lines, with pale centres and pale spaces between, over the whole body. The ground colour runs from purplish-blue through yellowish and greenish to orange-brown, and the fish changes colour and intensity quickly. The body is a rigid triangular bony box that cannot flex, so it swims by sculling with the dorsal, anal and pectoral fins and hovers. There is a pair of sharp horns projecting forward above the eyes and a second pair projecting backward from the rear of the box.
The scrawled cowfish is the only real confusion, and the check is simple: hexagons that close into cells here, open wandering scrawls there.
It lives in clear water around coral reef between 3 and 80 m, alone, slow, often head-down over the reef, feeding on sponges and other attached invertebrates. When stressed it releases a toxin from the skin that kills other fish in an enclosed volume; in open water it is harmless to everything, including divers.
There is an unresolved disagreement about the spelling of its species name. Poey described it as polygonius in 1876. Eschmeyer’s Catalog of Fishes and FishBase correct that to polygonium, on the grounds that the genus name is neuter and the ending must agree. WoRMS and most other databases keep the original. This site uses polygonius, which is the majority spelling and the original; both refer to the same fish.
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.