Banded crab living among the spines of sea urchins, gripping them with hooked claw tips. Usually one to an urchin.
A small crab of the Pilumnidae, carapace typically seven to ten millimetres across and slightly longer than it is wide — an unusual proportion among crabs and a useful first check. The ground colour is cream to pale pink, crossed by dark reddish-brown bands that continue onto the legs, and the carapace itself is smooth and hairless, carrying four prominent horns: one at each side and two between the eyes. The diagnostic characters are the large teeth on the upper edge of the legs and chelae, two on the merus and one on the carpus. These separate it from Zebrida brevicarinata, which has blunter horns and no teeth on the first three legs, and from Zebrida longispina, which has longer horns; both of those appear to be restricted to Australian waters, so a zebra crab on an urchin in Indonesian or Philippine muck should be this species. A maximum of 25 mm carapace width appears in some popular sources but cannot be traced to a measured specimen; published records sit between seven and ten.
Obligate on sea urchins, holding the spines with hooked tips to the walking legs. Confirmed hosts include the fire urchin Asthenosoma varium, Diadema setosum, Tripneustes gratilla, the flower urchin Toxopneustes pileolus, and species of Salmacis, Heliocidaris and Pseudocentrotus. There is normally one to an urchin: the crab is solitary from the time it settles, and although males move between hosts during the breeding season, crossing open sand to do so, pairs are transient rather than the resting state. It feeds on the host’s tube feet and surface tissue. It is found among the spines on the upper surface rather than underneath. Records run from the intertidal to beyond a hundred metres, though almost everything within diving depth is shallower than twenty-five. Unlike the Coleman shrimp, which shares the fire urchin with it, no host-matched colour change has been documented in this species.
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.
