A small urchin with five bare bands of blue skin, usually hidden under carried debris.
A small round urchin with a test to about 5 cm, banded vertically in ten alternating sections: five bare bands of velvety skin, iridescent blue, blue-green or nearly black, and five bands of short, dense, dark spines. Nothing else in the muck fauna is marked that way, and the identification is straightforward once the bare bands are seen. Juvenile Tripneustes gratilla is the usual mistake, being a similar size and also a coverer, but its spines are short, white and evenly distributed with no bare bands; small Salmacis are paler and likewise spined all over.
It is a habitual coverer. Shell fragments, seagrass blades, algae, scraps of plastic and whatever else is to hand are held over the upper surface by the tube feet, so what is actually on the sand is a small moving pile of debris with a fringe of spines under it, and that is why the animal is walked past. The covering is generally explained as shading, as camouflage, or as both. Grazing is on algae and surface film, mostly after dark, with the day spent tucked into rubble.
The spines are short and blunt and this is not an urchin that causes injuries. It is familiar from the aquarium trade under the tuxedo name; wild ones live on rubble, sand and the edges of seagrass, and are commoner on muck sites than the number of photographs of them suggests.
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.