Props itself on two locking pelvic spines and a stout dorsal spine, over soft bottoms at night.
A compressed, deep-bodied fish to about 30 cm, silvery on the flank and darker over the back, with a pointed snout and a small mouth set at the end of it. The three spines that give it its name are the tall first dorsal spine and a heavy spine in each pelvic fin; all three lock erect and can only be folded by releasing them, so a fish that has raised them holds a rigid triangular outline. The skin is rough, closer to sandpaper than to scales.
It lives over soft bottoms, more often silt and mud than clean sand, and it is an estuarine and coastal animal rather than a reef one. Feeding is on small benthic invertebrates picked from the surface of the sediment. Divers meet it mostly at night, in ones and twos, drifting just off the bottom on sheltered muck slopes; by day it is either deeper or in water too turbid to dive. Recorded through the Indo-West Pacific, and common in trawl catches well outside diving depth.
Juveniles are the animals more often photographed, at a few centimetres, translucent grey and with the spines already fully formed. The species is not one anybody plans a dive around, but it turns up often enough on silt to be worth recognising.
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.