Argonauta females secrete their case from two expanded arm membranes, unlike a nautilus shell, which grows from the mantle. The structure is calcite, brittle and thin, and it holds the eggs. For centuries its origin was disputed, with some naturalists arguing the argonaut had stolen it.
Julian Finn and Mark Norman resolved the buoyancy question in 2010 by observing females released in Japanese waters. Each rose to the surface, rocked the case to admit air, sealed the pocket with the arm membranes, and pushed down against the trapped gas until it was compressed enough to render the animal neutrally buoyant at her chosen depth. Animals prevented from taking in air struggled to maintain position.
Two older claims can be set aside. The membranes are not sails, and the argonaut does not sail on the surface as classical accounts had it. The males never build a case at all; they are only a couple of centimetres long and transfer sperm by detaching a modified arm, the hectocotylus, which was itself first described as a parasitic worm.