A sea snail that gave up the bottom, turned its foot into a swimming fin, and became glass.
Heteropods are predatory sea snails that live their whole lives in open water. The foot, which in a bottom-living snail is the muscular sole it crawls on, has become a single vertical fin that the animal beats while swimming upside down. The body is almost entirely transparent, so what is visible in a photograph is usually the gut, the eyes and the fin, with the rest inferred.
The Carinariidae keep a shell, but only a small glassy cap over the visceral mass, far too small to withdraw into and clearly on its way out — the related Pterotracheidae have lost it altogether. Carinaria cristata reaches half a metre and is one of the largest predatory gastropods in the ocean.
Most blackwater encounters are with larvae and juveniles of a few millimetres, at the veliger stage, when the animal still carries the coiled larval shell and the ciliated lobes it swims and feeds with. Larvae are not identifiable to species; the family is as far as a photograph will honestly take you.
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.
