Not an anemone at all, and the crab living in it is the reason to stop.
A cerianthid, in its own order and not a true sea anemone. It builds a soft felted tube of mucus and discharged threads, buried vertically in sediment, and extends two rings of tentacles from the top — long outer tentacles for capture and a shorter inner ring around the mouth. Crowns reach about 30 cm across and come in white, grey, brown, purple and a striking fluorescent green. Unlike a true anemone it has no basal disc and cannot move; withdrawn, it disappears into the tube completely and leaves nothing on the surface.
The reason it earns an entry is what lives in it. The harlequin crab Lissocarcinus laevis is an obligate associate, and tube anemones also carry commensal shrimps and occasionally juvenile fish sheltering in the tentacle crown. A field of them on open silt is a place to slow down. The animal retracts at pressure waves as much as at light, so an approach that puts a fin down two metres away closes the crown before the diver arrives. Recorded from about 3 to 40 m.
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.
