A slow urchin with a few thick blunt spines, each carrying a crust of growth of its own.
An urchin with a small number of stout cylindrical primary spines, blunt at the tip and often as thick as a pencil, set in widely spaced rows on a low test, with a ring of short flattened secondary spines around the base of each. Colour runs brown, fawn, grey-green and red-brown, sometimes with banded spines. The proportions are the giveaway: where the familiar urchins carry hundreds of fine spines, these carry a few dozen thick ones and the test between them is visible.
The primary spines of this group have no living tissue over them once they are grown, which is unusual among urchins, and the consequence is visible on every older animal. Algae, bryozoans, hydroids, small serpulid worms and sponge settle on the spines and grow there, so a large individual is carrying a garden and reads as a piece of overgrown rubble with legs. The spines are blunt and are not brittle in the way that the spines of Diadema setosum are, and they neither break off in a wound nor carry venom.
It is a slow nocturnal grazer, taking algae, sponge and encrusting growth, and it spends the day wedged under ledges, in rubble and in crevices. Prionocidaris and Phyllacanthus are the genera most often photographed in the region. Species are separated on the test plates and on the secondary spines, and records from a dive are normally kept at the order. Recorded from about 3 to 35 m, and more often found on the rubble and rock edges of a muck site than out on sediment.
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.