The false fire urchin: rigid test, rows of blue dots, and a reliable host for zebra crabs.
A large urchin with a rigid test to about 20 cm and long, slender, banded red-brown spines carried in five wedge-shaped groups. Between the groups the test is bare, and those five naked areas are the identification: red to purple-brown skin marked with curving rows of small iridescent blue dots, with a dark inflatable anal sac standing up at the centre of the animal. It is widely called the false fire urchin, and the comparison is with Asthenosoma varium, which has a flexible test that sags out of water, spines that carry a swollen bead of venom tissue below the tip, and no bare blue-dotted interambulacra. Juveniles are smaller, more strongly banded, and often found in loose aggregations on open sand.
It lives on soft bottoms rather than on reef, moving over sand and silt and grazing detritus and algal film, and it will shift ground overnight, so an urchin found on one dive is not reliably in the same place on the next. It is worth finding for its passengers. The zebra crab Zebrida adamsii sits on the bare test between the spine groups, usually flat against the skin where its stripes line up with the spines behind it. Urchin shrimps of the genera Stegopontonia and Periclimenes run along the spines, and cardinalfish of the genus Siphamia and other small fish hang in the spine canopy. The Coleman shrimp Periclimenes colemani is not one of them: it is an obligate associate of the fire urchins, and a shrimp of that pattern sitting in a cleared bare patch means Asthenosoma rather than this animal. Approaching from the side and low, rather than from above, is what makes the bare interambulacra and anything sitting on them visible.
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.