Argonauts are odd animals. They rather resemble a nautilus, but they aren’t particularly closely related to them; their closest cephalopod relatives are the octopuses. Females have a thin shell and scoot about in the water column, but the poor males are all dwarfs, rarely seen, with no shell.
What is the shell for? It seems to be a chamber for holding a bubble of air that the animals use to maintain neutral buoyancy. I’m a little surprised that this was a surprise, though — the analogy to the chambered nautilus is obvious, and all the photos and videos I’ve seen of them suspended in midwater suggested that they were maintaining neutral buoyancy somehow.