A shell-less heteropod: a long glass cylinder with a trunk at one end and a single fin underneath.
A heteropod that has given up the shell altogether. The body is a long transparent cylinder, almost invisible except for a trunk-like proboscis with the mouth at its tip, a pair of large eyes shaped like narrow triangles, a single vertical fin on the underside and a compact opaque visceral nucleus set well back. The animal swims upside down with the fin uppermost, driving itself along in slow undulations, and holds the proboscis out ahead.
The Carinariidae, which have their own entry, keep a small glassy cap of a shell over the viscera; Pterotrachea and its relatives have lost even that, and the visceral nucleus hangs free in the body. Both are predators. Pterotrachea takes salps, arrow worms, other heteropods and small fish, seizing them with the proboscis, and the gut contents are often the most opaque thing in a photograph.
Species are separated on the proportions of the fin, the nucleus and the tail, and most records from a photograph are best kept at the genus. Recorded from warm seas worldwide. It is one of the larger animals a blackwater dive produces and is normally seen as a length of faint refraction crossing the beam, with the eyes and gut arriving in the frame before the outline does.
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.
