A purple snail that hangs upside down at the surface from a raft of trapped air bubbles.
A thin-shelled snail with a globular shell of a strong violet to purple, deeper on the underside than on the top. It lives at the sea surface, hanging beneath a float of air bubbles coated in mucus that the animal builds itself: the foot is pushed up through the surface film, a bubble is trapped and wrapped in mucus that hardens, and the raft is extended as the snail grows. An animal separated from its raft cannot swim and sinks.
The colour is arranged for a life spent at the interface. Seen from below against the bright surface the pale upper shell is inconspicuous, and seen from above against dark water the deeper purple underside blends in, so the animal is countershaded in the reverse of the usual sense because it floats upside down.
It feeds on the floating siphonophores and hydrozoans of the same surface community, including Physalia and Velella, rasping at the float and the tentacles, and a snail is sometimes found attached to prey larger than itself. Egg capsules are carried slung beneath the raft in most species. On blackwater dives the animals appear at or just under the surface, usually at the start or the end of the dive, and they arrive in numbers after onshore wind rather than steadily through the year.
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.
