A scale worm that lives on sea cucumbers and takes the colour of the one it rides.
A polynoid scale worm, flattened, to about six centimetres, with the back covered by overlapping pairs of scales — elytra — and the segments carrying long club-tipped cirri that give the species its name. It is found only on sea cucumbers, and its colour is taken from the host: pale cream with dark blotches on a spotted Bohadschia, dark brown on a dark holothurian, so the same species looks different from one animal to the next. Confusion is with the free-living scale worms and with the bristle worms, and the practical separation is location — a scale worm on the body of a sea cucumber, moving with it, is almost certainly this species, since it is the only obligate holothurian polynoid commonly seen in the region.
It rides on the body wall and along the flanks, and moves onto the underside and around the mouth when disturbed, so an animal that seems to have nothing on it should be looked at along the ventral margin and near the anterior end before being written off. What it takes from the host is not fully settled; it feeds at least on the surface mucus and detritus, and there are reports of it grazing the host’s epidermis and tube feet, which puts it somewhere between commensal and mildly parasitic. Recorded hosts include Thelenota ananas, Bohadschia and Stichopus, and it shares those animals with the other sea cucumber associates — the emperor shrimp Zenopontonia rex and the sea cucumber crab Lissocarcinus orbicularis, both of which are more often on the underside. Working along a large holothurian slowly, rather than photographing it whole, is what turns up all three.
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.
