Hundreds of slender sea cucumbers grazing the surface of a single barrel sponge.
A slender synaptid holothurian to about 15 cm, translucent pale yellow to cream with fine brown longitudinal lines and speckling, and a ring of short branched tentacles at the front end. Like the other synaptids it has no tube feet and no respiratory tree, which is why the body is so thin and so extensible. What identifies it in practice is not the animal but the arrangement: it lives in aggregations on the outside of large sponges, most often barrel and tube sponges of the genus Xestospongia, and a single sponge may carry tens or hundreds of them draped over the rim and the outer wall. The related Synapta maculata is much larger, reaches three metres, is boldly striped, and lies over rubble and weed rather than on a sponge, so the two are not really confusable once the host is taken into account.
The association is the entry. The animals sweep their tentacles over the sponge surface and wipe them into the mouth, taking the detritus, bacterial film and suspended material that collects there, and the sponge appears to give them an elevated position in the current as well as a surface to graze. Whether they take anything from the sponge itself is not settled; the relationship is usually described as commensal, and the sponges carrying dense aggregations do not obviously suffer for it. They are most spread out and most photogenic in current, and they contract into a huddle if a diver comes in hard or backwashes them. The skin carries the same anchor-shaped microscopic ossicles as the rest of the family, so an animal that touches a glove sticks to it and tears rather than letting go.
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.