The Blanket Octopus

Welcome to our first installment of “Kortney’s Korner,” a place where Kortney, team member of the “So You Want to Be a Marine Biologist” podcast and MarineBio.life, dives into an oceanic topic of her choice. Have a suggestion? Email hello@marinebio.life or leave a comment below.

I don’t remember how I learned about blanket octopuses, but I do remember thinking, “This is why I love science so much.” As a student who staunchly avoided math and math-adjacent subjects, including science, up until last year, I am new in my exploration of the natural world. But every new discovery fills me with such delight, and serves as confirmation that I am making the right choice. I can pivot from a career focused on books to a career in the sciences. 

One such discovery is the snuggie of the sea, the ghostly creep of the briny deep: the blanket octopus. This animal is the embodiment of the phrase, “truth is stranger than fiction.” 

The blanket octopus belongs on a list with animals whose names are both fitting and intriguing, including: the red-lipped batfish; ice cream cone worm, which will make you think twice about stopping at an ice cream truck; and the fried egg jellyfish. 

The fried egg jellyfish is a tale for another time, because today, right now, it’s all about the blanket octopus. 

Blanket octopus is the common name for a genus of octopuses known as Tremoctopus. This genus consists of four identified species: T. violaceus, T. gracilis, T. robsoni, and T. gelatus. Once you’ve seen a photograph of this stunning cephalopod, you’ll understand how it got its common name. The females have a thin, partially see-through webbing attached to their dorsal and dorsal lateral arms, which can be seen flowing behind them like Egyptian cotton. 

The coloring of the blanket octopus varies from species to species – from a deep purple to a rainbow sheen – but the females all have the eponymous “blanket,” which they can unfurl at will. Because blanket octopuses live perpetually in the open ocean (cue The Chicks’ “Wide Open Spaces”), instead of at the bottom of the sea like most other species of octopuses, they can’t rely on camouflage or hiding to avoid predation. Which is why the blanket as a look-how-big-I-am intimidation factor is so necessary. If the size-enhancing blanket alone doesn’t work, the females can detach part of their impressive silky membrane to distract predators and protect their eggs. Blanket octopuses seem to have a penchant for dramatic flair, and even if their predators – like sharks and sailfish – don’t appreciate that, I certainly do. 

The female’s cape isn’t the only stark difference in appearance between the sexes. Female blanket octopuses grow to be up to six-and-a-half feet long, while the male blanket octopuses grow to about 0.9 inches long. This means the males are roughly the size of walnuts. This size discrepancy makes the blanket octopus a severe example of sexual size dimorphism in the animal kingdom. There is still a lot we don’t know about the blanket octopus, but scientists hypothesize that the size discrepancy has to do with the male octopus’ need for speed and not much else. 

The males don’t get their own tiny capes, but they do have a hectocotylus, aka a sex arm. That’s what it is colloquially called. In order to mate with females, the male blanket octopus detaches his hectocotylus and puts it in the female’s mantle, the mantle being the large, conical part of the octopus above its eyes that houses all its major organs and vaguely resembles a gnome hat. He wants to be fast about detachment and placement, because he’s competing with all the other male blanket octopuses. And competition is fierce: females can have sex arms from many different males in her mantle at the same time. They’ll stay there until the female is ready to fertilize her more than 100,000 eggs. Scientists believe that male blanket octopuses die shortly after detaching their hectocotylus. 

All of these aspects of the blanket octopus are fascinating, but one of my favorite facts I came across regarding the blanket octopus involves how it sometimes defends itself. Both male and immature female blanket octopuses have been known to use Portuguese man o’war tentacles as whips. It’s as metal as it sounds. They’re immune to the siphonophore’s sting, and can therefore get close enough to rip off its tentacles, which they then carry using the suckers on their arms. There are no records so far of females larger than 3 inches using this technique. Scientists have suggested various reasons as to why they no longer use this technique, including that the number of tentacles the female would need to adequately defend herself would be too heavy, or that the suckers of the larger octopuses are too big for them to carry the smaller tentacles.

There is still so much we don’t know about these absurd and beautiful creatures. I hope in the future there will be more research on blanket octopuses. But for now, I’m happy to know what I do about these animals that carry around stinging whips and wear majestic capes.