The largest of the argonauts, the female riding in a thin white case she makes with two of her arms.
An open-water octopus in which the female builds and carries a thin, brittle, translucent white case. In this species the case reaches about 30 cm and is the largest in the genus. It is narrow across the keel, and the keel carries two rows of small sharp tubercles. The winged argonaut, the species more often photographed in the strait, is smaller, browner and thicker walled, with a broader keel and fewer, blunter tubercles. The animal inside is a pale octopus with eight arms, two of them expanded into broad webbed flaps that are wrapped over the outside of the case and that secrete it.
The case is not a shell in the sense the nautilus has one. It is secreted by the arms rather than by the mantle, is not attached to the body, and functions both as a brood chamber and as a buoyancy device: the female takes air in at the surface, seals it inside with the arm webs, and uses it to hold neutral buoyancy at depth. Eggs are brooded inside on a stalk at the back of the case. Males are a different animal altogether, under about two centimetres, without a case, and are rarely seen.
Sightings are of drifting animals in open water, usually at night and usually well off the bottom, and females are sometimes seen riding jellyfish and salps. The record from these coasts is thin and irregular. Blackwater dives account for a growing share of it, and an argonaut on a night dive over a slope is a matter of being in the water when one passes.
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.
