The vast majority of octopus species live along the sea floor—whether that is in the sandy shallows off a tropical coast or in the dark depths around hydrothermal vents. But a handful of octopuses spend their lives swimming in the open seas, many using internal air-filled swim bladders to stay buoyant.

But females of one type of octopus have evolved an external engineering solution to stay aloft in the water column. Meet the argonaut octopus. The ladies of this genus (Argonauta) construct a paper-like "shell" case around themselves. Such an encasing is reminiscent of their cephalopod cousins the chambered nautilus. But those multi-tentacled creatures create hard, multi-sectioned shells and don't fare well outside of them.

The argonaut's shell is made of a thin calcite the female secretes as she matures. Aristotle decided that, "it uses this structure, when a breeze is blowing, for a sail." More recently, however, scientists speculated that its main purpose was to serve as a portable shelter for keeping eggs.

Its key as a swimming aid, however, was made clear in a 2010 paper in which researchers describe these octopuses in action, testing three A. argo specimens. "Female argonauts use the shell to 'gulp' a measured volume of air at the sea surface, seal off the captured gas using flanged arms and forcefully dive to a depth where the compressed gas buoyancy counteracts body weight," the researchers noted in their report. When the scientists removed the air from the octopuses' shells underwater, the octopuses tended to sink before swimming to the surface, taking in more air and then jetting back down to where they were weightless.

What about the males? They look like an entirely different genus altogether—and not only because they lack a shell. A. argo females, perhaps the largest species in the genus, grow to be about 50 centimeters long, but the males are about one-eighth the length and one six-hundredth the weight of the females.

To mate, the males surrender their hectocotylus, the detachable arm for transporting sperm. These small arms had long been found crawling on the females and were once thought to be a parasite, rather than a gift from a mate of the same species. In fact, it was reportedly arms found in female argonaut octopuses that 19th-century French naturalist Jean Cuvier mistakenly identified as worms (and named them for the hundred or so suckers each had and the hollow nature of their "bodies").

Once they are ready to lay their eggs, the females keep them in their shell until the young hatch. Which came first, the egg case or the swim aid? Although scientists have discovered plenty of fossilized argonaut shells, they have not yet discovered the genesis of this genus's adaptation.

Video courtesy of Cabrillo Marine Aquarium/YouTube

Illustration courtesy of Ivan Phillipsen