The female builds a paper-thin case with two of her arms and lives inside it.
An open-ocean octopus, not a nautilus and not related to one. The female secretes a brittle, translucent, paper-thin case from two greatly expanded dorsal arms and lives inside it, using it to hold her eggs and to trap gas for buoyancy; the shell is not attached to the body and she can leave it. Males are a fraction of her size, carry no shell at all, and are almost never seen. Argonauta hians has a brown-tinged case with two rows of low tubercles; A. argo is larger, whiter and more strongly keeled.
Encounters are a matter of luck and current. Females drift in the upper water column, often riding or clinging to jellyfish and salps, and reach a muck site inshore after a run of onshore wind or on a night dive over deep water. They are a standard blackwater subject and an unusual one on the bottom. Empty cases wash up and are sometimes found on the sand, which is not the same as finding the animal. Anything carried by an argonaut is being used as transport, so lifting one off its ride ends the encounter.
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.
