I just read MATSUMOTO and NIHONGI (Proc. Japan Acad., 55, Ser. B (1979), which is mostly about a Polyptychoceras specimen “set as an ornament at the entrance of a fish restaurant in Otaru (western Hokkaido), called Ginrin-so.”
They use another fossil to discuss mode of life and this is what they say:
“The specimen (about 200 mm in length in a restored condition) is probably an adult shell. Its body-chamber, comprising the last U-turned part, which is mostly dark coloured in the figure, is very long. The animal must have been very long like an eel. The volume of the body-chamber is somewhat larger than the total volume of the phragmocone. This must have been favourable for the benthic mode of life. Taking the septate part upside and the body-chamber below, Polyptychoceras could have kept a buoyancy, with some control through the siphon. Therefore, it may have taken an up-and-down locomotion to some extent. As the body-chamber is opened upward, the quick backward swimming by a jet-propulsion may be unlikely, if not impossible for the animal, but this situation must have been favourable for a slow and probably occasional up-and-down movements. To sum up, species of Polyptychoceras probably took a primarily benthic mode of life and were fond of sheltered places at the bottom of the sea-water.”
I'll read through the various buoyancy discussions (with their different assumptions about the size of the animal relative to body chamber) a bit later. Thanks for the links and I'm looking forward to understanding this better.