The hemichordate that makes many of the coiled sand casts on a muck site.
The animal itself is rarely seen. It lives in a U-shaped burrow in soft sediment, is pink, orange or fawn, and consists of a short muscular proboscis — the acorn — seated in a collar, with a long limp trunk behind it; lengths run from a few centimetres to more than half a metre. What is seen is the cast: a neat coil, or a low heap of extruded sand strings, at one end of the burrow, often startlingly regular. Digging one out generally destroys it, since the body wall tears under its own weight, and identification of Indo-Pacific enteropneusts rests on internal anatomy, so this entry is pitched at the class.
It is not a worm in the ordinary sense. Hemichordates are neither annelids nor molluscs but a small phylum sitting close to the echinoderms and the chordates, and the adults have pharyngeal gill slits, which is why the group takes up far more room in a textbook than it does on a dive. Feeding is either by swallowing sediment and stripping the organic content from it, which is what produces the casts, or by trapping particles in mucus on the proboscis and moving them to the mouth. Many species produce bromophenols and have a distinct iodine smell, noticeable in a bucket rather than in the water.
The cast on its own does not identify the maker. Sea cucumbers of the genus Holothuria leave similar coils on the same sand, and several polychaetes and echiurans do much the same, so a coil is best logged as a coil unless the animal at the bottom of it has actually been seen.
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.
