Urchin clingfish

A clingfish to about 5 cm, dark reddish-brown to nearly black, with a narrow pale yellow stripe running the length of each flank from the snout to the tail base and a pale mark on the caudal peduncle. The shape is the surprise. Most clingfish are squat and flattened against the surface they are holding; this one is long, thin and rounded in section, with a drawn-out pointed snout, and the ventral sucking disc formed from the pelvic fins is reduced compared with other members of the family. It is more often seen hanging free among spines than clamped to anything. Juveniles are thinner still, with the stripe proportionally brighter. Little else in an urchin resembles it once the shape registers, though at a glance among the spines it can be taken for one of the small cardinalfish sheltering in the same animal.

The host is a long-spined urchin. Diadema setosum is the usual one, and the banded urchin Echinothrix calamaris and its all-dark relative E. diadema serve as readily; both of those urchins have entries here. The fish orients head-down and parallel to the spines, which is what makes it hard to resolve, and it works around the far side of the test as a diver comes in. Feeding is partly on the host, taking tube feet and pedicellariae, and partly on small crustaceans, including the parasitic copepods living on the urchin, and on bivalve tissue. Adults are also found among branching corals and occasionally in crinoids and anemones, but an urchin is where the great majority are. Going through an aggregation of urchins on rubble or around pilings, with the light held off to one side, is how they are found: the fish shows as a straight dark line lying at a different angle from the spines around it.

Recorded from the tropical Indo-West Pacific, from Sri Lanka east through Indonesia and the Philippines to southern Japan and northern Australia, from about a metre down to twenty.