audiovideodisco writes "Even among octopuses, the Argonaut must be one of the coolest. It gets its nickname — 'paper nautilus' — from the fragile shell the female assembles around herself after mating with the tiny male (whose tentacle/penis breaks off and remains in the female). For millennia, people have wondered what the shell was for; Aristotle thought the octopus used it as a boat and its tentacles as oars and sails. Now scientists who managed to study Argonauts in the wild confirm a different hypothesis: that the octopus sucks air into its shell and uses it for ballast as it weaves its way through the ocean like a tiny submarine. The researchers' beautiful video and photographs show just how the Argonaut pulls off this trick. The regular (non-paper) nautilus also uses its shell for ballast, but the distant relationship between it and all octopuses suggests this is a case of convergent evolution."