Male and female are coloured so differently that they were long treated as two species.
To about 16 cm, with a rigid box of fused plates for a body and only the fins, jaws and tail able to move. Females and juveniles are black-brown covered in white spots. Males have a dark blue-black back with white spots, bright blue flanks with yellow-orange spots and yellow lines, and a blue tail; the two sexes were described separately in the nineteenth century.
Boxfish swim by sculling with the pectoral and dorsal fins and are slow, which is possible because the body is armoured and because the skin secretes a toxin, ostracitoxin, when the fish is stressed. That toxin is released into the water and is lethal to other fish in a confined space, which is why the family is a problem in aquaria and why a boxfish should not be handled or trapped. It feeds on sponges, tunicates and small invertebrates and is easy to approach and photograph. Recorded from about 2 to 30 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.