Okay, so the blanket octopus is a pelagic octopus, which basically means “sea-faring.” This is in opposition to, say, a benthic (lives on the sea floor) octopus or a littoral (lives very close to the shore) octopus. Quite frequently this also means “really hard to study, so fuck if we know.” Since the open ocean is a huge fucking place with lots and lots of comparative ecological dead zones, the pelagic lifestyle poses certain challenges. For one, there’s not really much in the way of shelter, so either good camouflage or good defenses are a must.
The blanket octopus is radically sexually dimorphic, with males being about the size of a peanut and females being about the size of a grown-ass adult human being. Both use ink to surprise and confuse predators if confronted, but the boys and very young ladies also steal chunks of Portuguese man o’ war tentacles (a pelagic siphonophore) to wave at their enemies. They’re immune to the stings themselves, and the tentacles can remain active for several months if kept in the water, but most animals experience the sting as extremely painful. So, you know, don’t fuck with a blanket octopus.
Above: Those thready little not-arm things he’s holding? You want none of them.
Adult females have a big ol’ honking sheet of brightly-colored webbing attached to two of their arms that they can unfurl to make themselves look approximately fifty times bigger than they really are if they’re confronted with something unpleasant, like a persistent NatGeo photographer. It also serves as a distraction, in the event that the apparent size increase doesn’t dissuade a predator. The webbing can be detached from the body and abandoned if need be.
A completely scientifically-unsupported but personally-enthusiastically-promoted hypothesis is that the detached webbing has the consistency of really thick peanut butter and the clinging capacity of saran-wrap, for added predator-stopping power. I mean, I just don’t think that something that grows up hitting other things with tentacles it ripped off a third thing suddenly just gets all zen about being attacked now that it’s big, you know?
When the boys happen to find a girl in their great oceanic trek, they tear off their jizz-arm, which attaches inside her mantle and hangs out until she’s ready to lay eggs, at which point it fertilizes her eggs. Did I mention that male octopuses (octopodes, octopi, etc.) totally have a jizz-arm? Because they totally do. The technical term for it is the hectocotylus, and it’s what most cephalopods rock instead of a dick. They don’t always tear it off—sometimes they keep it, sometimes it breaks off of its own accord during the mating process—but a lot of them do. Ha ha nature is fucking horrifying, remind me to tell you about mating in honeybees sometime.
In other horrifying news, it’s called a hectocotylus instead of a jizz-arm because the researcher who discovered it found it already inside a female and thought it was a parasitic worm, not a dong. It wasn’t until they noticed that the guys had it as a normal part of their anatomy that they realized what was going on. Seriously, guys, cephalopod sex. Gives a whole new meaning to the phrase “dick in a box.” They’re like, “Hey, baby, I got you something!” and the ladies are like “Oh my god you got me your dick! Just what I always wanted! Gimme a second to just stuff that right up my gill-slit for future use!” and then human scientists are just like “What the fuck, you guys.”
There might even be a case study somewhere combining cephalopod puzzle-solving and cephalopod detachable jizz-arms called like “Spatial Manipulation and Hectocotylic Choice by Females in the Absence of Males (Octopus vulgaris): Cognitive Demands and the Influence of Mate-Choice,” but you probably go on a list if you punch it into a database under those search terms.
Anyway, in the blanket octopus, once the boy’s found a girl, ripped off his jizz-arm, and given it to her, he just sort of wanders off and dies. This isn’t necessarily from the injury; octopuses lose limbs all the time and are perfectly capable of regrowing them. It’s just pretty typical in both sexes of most cephalopods to kick the bucket right after mating (for boys), or right after egg-laying or egg-hatching, depending on the level of maternal care exhibited by the species (for girls). Rather than laying eggs in a den and guarding the nest-site, like a lot of less nomadic species, lady blanket octopuses just glue tens of thousands of eggs to their bodies and carry them around until they hatch.