Image: Fin et al. Museum Victoria

At first glance, you may think you've seen this animal before – but look again. It resembles a chambered nautilus, the shelled mollusc we encountered off the Palauan coast recently, but this is an entirely different creature. It's an argonaut, a type of octopus that's unlike any bottom-dwelling species.

Few people in the world study the mysterious cephalopods, who cruise the world's oceans suspended in the water column, tucked away inside their delicate shells. Encounters are rare, so when we stumbled across footage of an argonaut school, we reached out to one such scientist: Dr Julian Finn of Museum Victoria. Much to our surprise, he'd never seen anything like it.

"They're all female," explains Finn, who, along with his colleagues, discovered that the paper-thin casings you see in the video help keep argonauts afloat. Male argonauts measure in at just a centimetre in length, and lack the enlarged, web-like tentacles used to secrete calcium carbonate shells. "I believe female argonauts spend their entire lives in congregations like this," he adds. "But this is the first video I've seen showing a wild aggregation."

At just 225 microns – about twice the width of a human hair – the shell is the thinnest secreted by any mollusc. It's thanks to this beautiful structure that the animals got their misnomer of a common name: the paper nautilus.

The male argonaut. Image: Finn, Museum Victoria

"In the early days people didn’t really understand what was going on," says Finn. "They would find an octopus within this shell, and these long webs. A mythical story began around the time of Aristotle that says the argonaut female actually lived in the shell and raised those webs as sails, to sail across the oceans."

It was with this legend in mind that famed naturalist Carl Linneaus gave the species its Latin name, which roughly translates to "swift sailor".

The function of the shell was debated for millennia, the truth muddled even more by those who speculated that the octopuses were mere parasites who fed on the shells' original inhabitants. "People believed that octopus and shell didn’t go together," explains Finn. 

By "gulping" air at the surface, the animals are able to use their shells as makeshift ballasts, much like the chambered nautilus does. But unlike their very distant cousins, argonauts are able to pop out of their shells at any time.

Diver Dam Nguyen, who encountered this aggregation off the Californian coast, was quite surprised at the animals' agility. "When the camera got too close, several of them latched onto [it]," he recalls. "They needed to be peeled off the camera body and returned to the pod."  

It's likely that argonauts stick together for the same reason that other marine species do: there's safety in numbers. Moving in unison can discourage predators by giving prey the appearance of a single, much larger creature, and makes it difficult to pick one off from the bunch. It's also possible, however, that the behaviour has something to do with the argonauts' strange sex habits. 

When it's time to mate, the male argonaut will detach his hectocotylus, a worm-like arm that doubles as a takeaway sperm packet. The female will use his dismembered member to fertilise her eggs, which sit safely inside their paper crib. It could be that females in the group who don't find suitable mates benefit from any discarded donations.

The detached, free-swimming hectocotylus of the chambered nautilus

We're only just cracking the shell of argonaut biology, but one thing's for certain: encounters like the one captured by Nguyen offer a unique view into an unseen world, one that might not be around forever. As our oceans continue to warm, increasingly acidic waters will erode the shells of marine creatures like the paper nautilus. 

To find out more about these enigmatic animals, Finn and his colleagues have called on the public to report argonaut sightings. Find out how you can help on the Argosearch website.