Meet the Argonaut, The World’s Weirdest Octopus

© Comingio Merculiano / Wikimedia Commons

Octopuses are awesome.

These eight-legged oddballs of the ocean have always had a dedicated fanclub, and the recent documentary My Octopus Teacher helped millions more people fall in love with them. And yet, I’d argue that anyone but the most artent of octopus lovers fails to appreciate the sheer diversity (and weirdness) within the cephalopod world.

There’s the blue-ringed octopus, which has enough venom to paralyze 10 people. The mimic octopus impersonates other sea creatures, while the coconut octopus carts around discarded coconut husks for protection. But perhaps the weirdest of them all is the argonaut, or paper nautilus.

Mysterious and misunderstood, the argonaut is unlike any other octopus species on earth. And as you’ll soon learn, there’s still so much we don’t know about these creatures.

An argonaut shell. © DBILIJ MUSTAPHA / Wikimedia Commons

They’re Not A Nautilus

First of all, the ‘paper nautilus’ isn’t actually a nautilus. Also known as argonauts, these creatures derive their name from the paper-thin, spiralled shell that females produce to shield their eggs. 

And while argonaut and nautilus shells may look like, that’s where the similarities end. Argonauts are a type of octopus, while the nautilus is, well, a nautilus. Both species are cephalopods, the taxonomic class comprising all octopuses, cuttlefish, squid, and nautiluses. 

Argonauts happen to be the world’s only pelagic octopuses. Instead of living near a structure on the seafloor, like a rocky shoreline or coral reef, like most octopus species, argonauts spend their lives floating near the surface of the open ocean.

The genus Argonauta derives its name from Greek mythology. The Argonauts were the famed sailors of the ship Argo, who helped Jason on his quest to recover the Golden Fleece. Early naturalists thought that argonauts ‘sailed’ around the ocean using two of their tentacles, hence the name… and a lot of weird anatomical postures in early drawings of argonauts. (In reality, argonauts scoot about by expelling water through their funnels.)

© Gerard Van der Leun / Flickr

There are a handful of species in the Argonauta genus, with most experts recognizing four unique species. (Argonaut shells are highly variable, creating a lot of confusion and scientific debate into the exact number of species.)

Two of those species, A. argo and A. hians, are found worldwide in tropical and subtropical oceans. A. nodosa lives in the Indo-Pacific and along the west coast of South America, while A. nouryi is found along the western coast of North America. 

All argonauts are sexually dimorphic, which is just a fancy way of saying that males and females look dramatically different. Females of the largest species, A. argo, have a mantle (main body, not the tentacles) that’s about 5 inches long, while their shells can be as large as 12 inches. The males are just a fraction of their size, measuring ¾ of an inch. 

Illustration from Natural History: Mollusca, 1854. © Phillip Henry Gosse / Wikimedia Commons

Males Dismember Themselves to Mate

Naturalists used to think argonauts were like hermit crabs, commandeering the shells of other creatures. Then in the 1830s, a French seamstress-turned-naturalist named Jeanne Villepreux-Power conducted rigorous experiments on argonauts and discovered that females create their own shells by secreting a calcite substance from two elongated tentacles. 

After creating her egg case, the female gulps an air bubble at the surface and seals it into the shell with her tentacles. Then she dives down until the weight of the compressed air cancels out the weight of her shell, making her neutrally buoyant in the water column. (Nautiluses, on the other hand, have shells with built-in air chambers.) Then the female argonaut deposits her eggs inside the shell and waits for a male to fertilize them.

The shells of various argonaut species. © Mgiganteus1 / Wikimedia Commons

Now here’s where things get weird. 

Male argonauts use a modified arm, known as a hectocotylus, to transfer their sperm to the female. This special tentacle, found in a pouch under their left eye, has small grooves to hold the sperm securely in place. But the males don’t just reach out and pass off a parcel of sperm. Oh no. They detach their own arm and give it to the female. 

The female inserts the hectocotylus into her egg case, holding onto it until she’s ready to fertilize her eggs. The male dies, or so we think.

Scientists weren’t aware that male argonauts existed until the 19th century. Knowing little about the species reproduction, they thought all argonauts were the larger, shell-building females. Famed naturalist Georges Curvier observed the dismembered arms and thought they were a species of parasitic worms, which he named Hectocotylus octopodis. Worms they were not, but they still bear the name hectocotylus.

We know a bit more about argonaut reproduction today, but scientists still have never observed live, male argonauts in the wild. 

Curvier’s original drawing of a hectocotylus. © Georges Cuvier / Wikimedia Commons

We still know relatively little about argonauts. 

Scientists aren’t sure why they tend to wash up on beaches in mass strandings every few years, or why argonauts sometimes ride jellyfish or seaweed, or grouping together in large rafts. And how do males and females find one another in the open ocean? How and why did they evolve their shells?

While these and many other questions remain a mystery, one thing is for sure: the argonaut is definitely the world’s weirdest octopus.

A paper nautilus hitches a ride on a red bell sea jelly on Ningaloo Reef, Australia.© Meg Green/TNC Photo Contest 2019
Justine E. Hausheer

Justine E. Hausheer is an award-winning science writer for The Nature Conservancy, covering the innovative research conducted by the Conservancy’s scientists in the Asia Pacific region. She has a degree from Princeton University and a master's in Science, Health, and Environmental Reporting from New York University. Justine's favorite stories take her into pristine forests, desolate deserts, or far-flung islands to report on field research as it's happening. When not writing, you can find her traipsing after birds, attempting to fish, and exploring the wild places around her home in Brisbane, Australia. More from Justine

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