The Beautiful Octopus Whose Sex Is All About Dismemberment

What are you doing in that shell, little octopus? You don’t belong in a shell. Go home. You’re drunk.

In Jules Verne’s 20,000 Leagues Under the Sea, the crew of the Nautilus takes a break from going for swims and pissing off native islanders to admire an armada of strange beasts floating along the surface, swimming backward by blasting water out of their bodies. “Six of their eight tentacles were long, thin, and floated on the water, while the other two were rounded into palms and spread to the wind like light sails,” notes our trusty narrator.

It’s an actual animal, alright, a rare and bizarre octopus that swims the open ocean: the argonaut. But those arms are no sails. (The myth persists in its name, though—the Argonauts being the Greek heroes who sailed around having sweet adventures.) The female argonaut uses them to form what is surely one of the most beautiful animal-made structures on Earth, an exceedingly delicate shell that she uses not to protect herself, but to house her young out there in the treacherous expanses of the sea. The males, though—things aren’t so great for them. Like, a sexual-dismemberment type of not so great.

Incredibly, females are up to 600 times the weight of the diminutive males, which grow to less than half an inch long and never build shells. According to marine biologist James Wood, though, there’s still a whole lot we don’t know about this mysterious creature, and its mating habits are no exception. What we do know is that when the male and female hook up, the male leaves something behind—other than sperm, of course. Specifically, it’s a specially adapted arm called a hectocotylus, “and there a little grooves on the arm that the sperm travels down and goes into the female’s oviduct,” Wood said. “So it’s not the penis. The penis is sort of inside the male, but that reproductive arm is used to transfer the sperm inside the female.”

The argonaut’s gorgeous shell posing on a beach all seductive-like.

When he jets away, the arm snaps off and stays with the female. It’s not clear if the male dies after copulation, but if he survives, the arm would probably grow back, because octopuses happen to be good regenerators. Some females have even been found with multiple hectocotyli in their shells from multiple males. Unsurprisingly, this odd sex caused quite some confusion for early observers. The famed 18th- and 19th-century naturalist Georges Cuvier determined the hectocotylus to be a species of parasitic worm, seeing things like a digestive tract that most certainly weren’t there (the name hectocotylus means “hollow thing”).

While almost all octopus species lay their eggs in crevices and caves and such, as an open-ocean swimmer, the argonaut has no such luxury. So as she grows, the female continually builds her shell—which is so thin it’s translucent—with those rounded arms, which secrete the mineral calcite. And when she’s finally ready to have kids, she’s got nice little crib for them. “So they can put them in this shell and they’re protected from the outside,” Wood said. “And the octopus can take care of them and brood them and clean them, make sure there’s no parasites, and oxygenate them.”

Argonauts can pop out of their shells whenever they like, unlike sea turtles. Also unlike sea turtles is pretty much everything else about the argonaut.

That shell serves another rather more cryptic purpose. I don’t want to tell them how to live their lives, but octopuses are supposed to be crawling around the sea floor, not floating around in the open ocean, and this presents the problem of buoyancy. Fish solve this with a gas-filled sac called a swim bladder, as do at least two other species of open-ocean octopuses, but the argonaut has a different solution.

By periodically coming to the surface and “gulping” air into its shell, the female can “seal off the captured gas using flanged arms and forcefully dive to a depth where the compressed gas buoyancy counteracts body weight,” wrote the researchers who discovered the function in 2010. And as her eggs grow and get heavier and heavier, she’ll be able to adjust for the added weight in this way. This is vitally important for a free-swimming octopus because constantly adjusting to maintain position sucks up a whole lot of energy. By becoming neutrally buoyant, the argonaut can kick back and relax.

The argonaut’s distant cousin, the nautilus.

The argonaut’s distant cephalopod cousin, the nautilus, accomplishes the same thing with its own shell, which is much thicker so as to function as armor and is permanently attached, whereas the argonaut can pop out of its shell whenever it likes. In fact, all cephalopods descended from shelled ancestors, but something happened over the course of their evolution that made them evolve away that shell—save for the nautilus, which is truly an ancient species. (The argonaut is also known somewhat confusingly as the paper nautilus, but again, they’re only distantly related.)

It may have been the case that big rude predators came along and were strong enough to break through those shells. “I use this analogy that we had knights and they would wear plate mail armor, so you had big heavy guys with big heavy armor and that was pinnacle of military combat,” said Wood. “And then we invented rifles and now a guy in heavy plate mail armor is a big slow target, because the ball is going to go through the plate mail, and it made it obsolete.”

Cephalopods were forced to adapt and develop new strategies or face extinction, and today we see them diversified into all manner of forms. While the argonaut’s shell is worthless as armor, the octopus still packs that iconic ink defense. Another species, the mimic octopus, seems to imitate other creatures you wouldn’t want to mess with like sea snakes and lionfish. And cuttlefish are absolute masters of camouflage, somehow blending into pretty much any environment even though they’re colorblind, which is…problematic if you’re a fan of reason. And almost all cephalopods have evolved an intelligence that is at times startling.

What’s interesting about the argonaut is that its ancestors long ago evolved away the shell, then later on evolved an entirely new way of constructing it and an entirely new use for it as a reproductive aid. Still, though, it arrived at the same solution to the buoyancy problem that the early cephalopods solved with their own shells, which just goes to show what an endlessly creative force evolution is.

Perhaps I’m making the argonaut seem like a helpless floating piece of meat, though. Like any other cephalopod, this is a voracious carnivore, targeting all kinds of crustaceans and sea slugs and other free-swimming prey. Once it’s got a hold on the quarry, the argonaut gnaws into the victim with its beak and injects a toxin (the chemicals at work here differ between cephalopods, and it’s not yet known what’s in the argonauts, according to Wood). Then, using a conveyor belt of tiny teeth known as a radula, it’ll scrape away the meat—or in the case of a hard-shelled prey, scoop out the contents through the hole created by the beak.

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