A triangular boxfish with a spined ridge along the back and no horns above the eyes.
A boxfish to about 30 cm, though most seen on muck are a third of that, enclosed in a rigid carapace that is triangular in cross-section: flat-bottomed, with the sides rising to a sharp ridge along the back. The ridge carries a strong spine pointing backwards over the middle of the body, and there is a short spine at each lower rear corner of the carapace. Colour is pale grey-green to yellowish-brown with small dark spots, sometimes with a bluish cast on the flanks. There are no horns above the eyes, which separates it from both cowfish, Lactoria cornuta and Lactoria fornasini; the yellow boxfish Ostracion cubicus has a squarer section and no dorsal ridge at all.
It works open sand, silt and seagrass rather than reef, hovering just clear of the bottom on small sculling fins with the rigid body held level, and feeds on small invertebrates taken from the sediment along with algae and seagrass. Juveniles are rounder and more heavily spotted and are the ones usually found on shallow muck; larger fish are more often in seagrass.
Recorded from about 1 to 30 m. Like the rest of the family it does not flee so much as drift away, and a slow parallel approach holds it far longer than a direct one.
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.