An argonaut is also known as a paper nautilus. It's a cephalopod, but it's not a nautilus. Nor is it a squid or cuttlefish, but it has a shell. Argonauts are octopuses with shells. And in case you are wondering, the genus Argonauta was named after crew of the mythical Greek ship. Aristotle, Jules Verne, and others have written about these creatures using their shells like a boat to sail across the sea and their tentacles as sails to catch the wind. That's a myth, but the truth about argonauts is even weirder.
The shells they make are not related at all to other cephalopod shells. The argonauts paper-like shells were developed independently after shell-less cephalopods evolved. But are these shells egg cases, helmets, or flotation devices? The argonauts scoop air into them to make themselves buoyant, which lends some credence to Aristotle's assertions, just with different anatomy. So far, the argonauts are strange even among octopuses, but wait until you hear about their bizarre sex lives. The video is less than ten minutes long, the last part is an ad. -via Laughing Squid