Red urchin with venom sacs set below its spine tips, and the fire urchin the commensal shrimps and crabs are named for.
A large urchin, roughly 20 to 25 cm across, red to purple-red, and unmistakable once the spines are looked at closely: each aboral spine carries a swollen, bead-like sac of flesh just below its tip, pale blue-white against the dark body. Nothing else in the muck fauna carries that structure, and it is the whole identification. The test is flexible rather than rigid — a family character, and the reason the animal collapses under its own weight if lifted out of water — and the oral spines end in expanded hoofs for walking over soft sediment. The common confusion is with Astropyga radiata, widely called the false fire urchin, which has a rigid test, long unswollen spines in five vertical clusters, and bare red skin marked with rows of iridescent blue dots. Fire urchins in the Red Sea are Asthenosoma marisrubri, a separate species since 1998; WoRMS marks the old Red Sea records for A. varium as inaccurate.
The venom is delivered by the spine sheath rather than by pedicellariae, the spine tapering like a needle and perforated with pores along its shaft. It stings and the pain is reported to last some hours. That is as much as can honestly be said: no toxin from the genus has been chemically characterised, the delivery mechanism is described in the primary literature as hypothesised rather than demonstrated, and there appears to be no published clinical case for any Asthenosoma. Claims that stings are life-threatening come from the aquarium trade and have nothing behind them. Mostly nocturnal, spending the day deeper and moving shallower after dark, and capable of travelling faster than an urchin has any right to. It is worth finding for what lives on it: Brock’s urchin shrimp Allopontonia brockii, the zebra crab, and the Coleman shrimp, which clears a bare patch of spines to sit in. One caution on that last association — the Coleman shrimp was described in 1975 from Asthenosoma intermedium in Queensland, and although virtually every photograph of it in the Coral Triangle is captioned as this species, no published source confirms A. varium as a host. It is safer to call the shrimp an obligate associate of fire urchins than of this 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.