The Octopus That’s Pretty Much Just a Swimming Blanket

The female blanket octopus will sadly enough never know the comforts of an actual blanket.

Odds are that as a kid you had what is known as a “security object,” perhaps a stuffed animal or toy that you carried everywhere with you. I had a plushy named Mr. Bear because I was apparently a painfully uncreative child, my sister had one named Beary because she was a little bit more creative, and Linus from Peanuts had his famous blanket.

But roaming the open oceans is a creature that sports a true security blanket: the so-called blanket octopus. Stretched between the cephalopod’s highly elongated arms are vast sheets of flesh, and when the octopus feels threatened, it splays out its arms to deploy a stunning cloak that maybe, just maybe, will convince an encroaching predator to piss off.

Even if that doesn’t do the trick, the octopus’ arms will break off in its enemy’s mouth, like a lizard losing its tail, hopefully allowing the erstwhile prey to beat a retreat. These are invaluable defenses because unlike their cousins, blanket octopuses don’t ever spend time on the seafloor, and thus don’t have the luxury of crevices to squeeze into for protection. And that’s very weird for an octopus.

But perhaps the strangest thing about the blanket octopus has nothing to do with its weaponry or flowing cape. While females can grow to a formidable 6 feet long, males are incredibly hard to find. They’re so small—less than an inch long—that they could fit inside the ladies’ pupils, according to Tom Tregenza, a biologist at the University of Exeter who co-authored a 2002 paper describing the first-ever sighting of a live male blanket octopus. A female blanket octopus can be an astounding 10,000 times heavier than a male, perhaps up to 40,000 times. Indeed, the differences between the two sexes are more extreme than any other animal this size.

But before we dive into that, let’s first talk about one of the gutsiest weaponizations in the animal kingdom. The diminutive males don’t have the flashy flesh blankets of the females, but they can exploit a menace that other creatures, including humans, would be mad to mess with: the infamous Portuguese man o’ war. The blanket octopus is immune to its stings, so a male will just waltz up and snag a tentacle (the man o’ war isn’t actually a jellyfish, but a sort of colony of clones known as a siphonophore). The little male octopus then wields the tentacle like a toxic whip to fend off predators. And that, as biologists say, is legit. Juvenile females also do this, but abandon the strategy as they grow into giants, “because once you get very large, it’s not much of a defense” trying to sting your predator’s mouth, says Tregenza. “If a shark is going to take a big bite out of you, it’s probably not going to worry very much about the peppery taste.”

So why exactly have male blanket octopuses evolved to be so tiny? “I think the males are small because they’ve got no real reason to be big,” he says. “Females have an obvious reason. Females are generally bigger in most animals because if they’re bigger, they can make more eggs or babies, and the more babies or eggs you can make, the more chance you have of getting your genes into the next generation.” But male sperm? Tiny. So males can afford to be miniscule.

Physical differences like this are known as sexual dimorphism. You sometimes see it in something like size or ornamentation—the female bird of paradise is positively drab compared to the flamboyant male. But females are typically bigger than males not only because their gametes take up a lot more space than sperm, but because eggs take tremendous energy to produce. And while males can get away with being tiny, the trend tends to reverse itself when males compete fiercely for females. In that case, bigness can be an evolutionary advantage, since the largest males win the right to mate. Thus does the human “dude bro” seem to get bigger and bigger as the years go by.

I’d be willing to bet that the male blanket octopus has some serious inadequacy issues.

Interestingly, the blanket octopus’ radical brand of sexual dimorphism is actually more common than you’d think in the open ocean. Another species of octopus, the argonaut, has tiny males who fertilize giant females (which build a beautiful shell out of calcite to house their young). The deep-sea anglerfish has similarly famous sex antics. The eensy-weensy male bites onto the female and permanently fuses his face to her flesh, releasing sperm whenever she demands it for perhaps decades. Finding females is just so difficult for these males that they evolved to do pretty much nothing but mate, so there’s no need for them to be any bigger than they have to be.

So the tiny male blanket octopus finds himself floating out there all alone in the vast ocean. He’ll be lucky to find a female, but once he does, he leaves his mate something special to remember him by—like, uh, a limb. “The blanket octopus male puts all the sperm its got into a modified arm,” says Tregenza. “The arm then breaks off and crawls into the female’s mantle cavity.” There, it may even find company: Females can retain the arms of multiple males simultaneously. The male then jets away, though the female isn’t necessarily fertilized just yet. “The female blanket octopus will have the male’s arm inside her,” Tregenza says, “and when she comes to need to fertilize her eggs, she can pull that arm out and squirt the sperm over her eggs like squirting soy sauce onto fried rice.” Thus concludes the greatest analogy in the history of science.

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