A crown of feeding filaments on a soft tube, withdrawn the moment a shadow crosses it.
A sabellid polychaete living head-up in a tube built of mucus and cemented sediment, of which only the crown is normally visible: two semicircular fans of feathered radioles, banded in brown, maroon, cream or white, spreading to about 10 cm across. The useful separation from the Christmas tree worm Spirobranchus giganteus is the tube and the plug. A serpulid builds a hard calcareous tube bored into living coral and closes it behind itself with an operculum; a sabellid builds a soft, flexible, sand-crusted tube standing in rubble or sediment and has no operculum at all, so a withdrawn feather duster leaves an open tube end. Species-level identification in this genus is unresolved — several nominal species are separated on characters that are not visible in the field — and the entry is pitched at the genus for that reason.
The crown does two jobs. Cilia on the radioles drive a water current through the fan and carry trapped particles down grooves towards the mouth, where they are sorted by size: the coarsest are rejected, the intermediate grades are stored and used as building material for the tube, and only the finest are eaten. Withdrawal is triggered by shadow and by pressure waves and is close to instantaneous, and the worm stays down for a minute or more before the crown creeps back out, so the working method is to set focus at a distance, wait out one cycle without moving, and take the frame as it reopens. Crowns cropped by fish are regrown. The tubes themselves are worth a look: they collect hydroids and small algae, and the base where the tube meets the rubble holds small shrimps and brittle stars.
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.
