An unsegmented worm that pulls its entire front end inside itself and becomes a smooth lump.
A soft, unsegmented worm with a body in two parts. The trunk is a smooth or finely warty sac, fawn, pink-grey or brown, and the front section, the introvert, is narrower and carries the mouth and a ring or crown of short tentacles at its tip. The introvert is pushed out to feed and pulled back inside the trunk when the animal is disturbed, turning outside in as it goes, and a fully retracted worm is a plain rounded lump with no head, no segments and no appendages, which is where the name comes from. Sipunculans are their own phylum and are neither annelids nor echiurans.
Most live buried in sand and silt, in crevices in rubble and dead coral, in old bivalve borings and inside sponges, and on a muck site the usual sighting is of the introvert extended a few centimetres from a hole or from the underside of a piece of rubble that has been turned. Feeding is on deposited material, either swallowed with the sediment or picked up on the tentacles. Withdrawal is fast, and once the animal is in there is nothing at the surface to mark it.
One sipunculan on this site is seen constantly without being recognised as a worm at all. Aspidosiphon corallicola lives inside the walking coral Heteropsammia cochlea, occupying the gastropod shell the coral has grown around, and it is the worm that drags the whole assembly across the sediment. Identification of Indo-Pacific sipunculans rests on the hooks on the introvert and on internal anatomy, so records from a dive are properly kept at the phylum. Recorded from the shallows to about 30 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.
