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All octopuses start out as teeny, tiny plankton, and most grow up to settle down on the seafloor. The blanket octopus, however, never settles down, and spends its life wandering the open ocean.

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[♪ INTRO].

All octopuses start out as teeny, tiny plankton, drifting through the ocean at the mercy of currents. Eventually, though, they get big enough to settle down and make a home for themselves on the seafloor... but not the blanket octopus.

The four species in the genus Tremoctopus never settle down. Though they get quite big, they spend their lives wandering the open ocean. And not even scientists know very much about these nomadic invertebrates.

But what they have found out is amazing! It turns out they have some pretty weird traits that help them survive the high seas— including those blanket-like appendages they're named after. Most octopuses spend a lot of their lives hiding, whether it's in rocky dens, holes in the sand, or large shells.

They only swim around when hunting or defending their territory —and even then, they'd prefer to crawl. Which I guess makes sense, since they have no bones to protect their soft, squishy bodies. The only hard part of an octopus is its beak —the calcified mouthparts it uses to eat its prey.

And blanket octopuses are also mostly soft. But still, they don't seek protection on the seafloor. They are generally found cruising through warm, tropical waters around the world.

And they can do that because they have all the usual octopus defenses—like the ability to release ink and change color at a moment's notice— as well as a couple of extra tricks. There's that namesake blanket, for instance. It refers to the long transparent webs of skin which connect four of a female's arms.

When threatened, she is able to unfurl all this skin and trail it out behind her like a superhero's cape. And in the largest species, she can grow to be over 2 meters long. So when you combine her size with her incredible cape, she looks pretty intimidating!

If this doesn't scare off a would-be predator, she may be able to ditch the blanket instead. Lone blankets have been spotted by divers, and some experts believe that's because a female can break it off much like a lizard does its tail when threatened. The idea being that, hopefully, that distracts the predator long enough for her more essential bits to escape.

And when the animals are smaller, they have another trick they can use. They're actually immune to the stinging cells of Portuguese man o' war and their close relatives. A young female blanket octopus will swim up to a man o' war, rip off its tentacles, and then carry them around using her suckers.

Then, when a threat comes too close, she can wield them like toxic whips! It's something the unfortunate scientist who first published this behavior discovered firsthand. Now the males steal tentacles to defend themselves, too.

But they don't just wield them when they're young because… well… they never get big. In fact, they're so tiny that scientists didn't observe them alive until 2002. The males of the largest species only grow to be about 2.5 centimeters long!

On average, males are one one-hundredth the length and 40 thousand times less massive than their mates! That's the most extreme size difference between sexes that we know of among non-microscopic animals. Just imagine if your spouse were the size of a walnut.

Scientists think that the males evolved to be so small because all they really need to do is mate. You see, all male octopus have a special appendage called a hectocotylus—what is sometimes totally seriously referred to as a “sex arm”—that they can fill with packets of sperm. In most other species, a male sticks this sex arm into a female's mantle cavity during courtship to deposit his sperm packets—a process that can take several hours.

But in the open ocean, there's zero privacy, so romance kind of goes out the window. When a male and female blanket octopus meet, he just rips off his sex arm and quickly sneaks it into her. The arm crawls on its own up deeper into the female —where it might find a few other arms already hanging out.

Eventually, the female will run out of space. So for males, it's not size that matters—it's speed. And growing big takes time, so larger males are at a disadvantage.

Once a male has delivered his sex arm, he dies. His work is done. Then, when the female is ready, she picks the arm she likes best and squeezes the sperm all over her eggs.

If she lived on the bottom like other octopus, she'd probably lay these eggs in her den so she could care for them until they hatch. But, since the open ocean lacks adequate housing, she constructs a cigar-shaped anchor rod for them out of calcium carbonate —the same stuff in shells and coral skeletons. And around 100 thousand eggs can attach to this rod, which hangs on one of her arms, keeping them all safe while she continues to move about the ocean.

It's thought that she dies shortly after the babies hatch, mostly because that's what other female octopus do —but researchers don't know for sure. In fact, we have a lot left to learn about blanket octopus, including how they find one another in the vastness of the ocean, or whether they sleep. In the meantime, we can appreciate these bizarre animals and the incredible ways they boldly go where other adult octopuses wouldn't dare.

Thank you for watching this episode of SciShow! And a special thank you to our President of Space, Matthew Brant. Matthew is one of the awesome people that supports the show through Patreon.

So he helps ensure the team here can keep making free, educational science content. And that, in our completely scientific and totally unbiased opinion, makes Matthew pretty rad. If you want to learn more about our wonderful patron community, you can head over to [♪ OUTRO].