Severed Octopus Arms Have a Mind of Their Own

Octopuses are renowned for their smarts (they can open jars!), and most of their 130 million IQ-raising neurons are located not in their brains but along their eight tentacles. Researchers think this allows octopuses to become the ultimate multi-taskers, Katherine Harmon, who’s got a book on octopi coming out soon, writes at Scientific American, since each of their arms can busily work away at some pesky mollusk shell or feel around in some new corner of habitat, nearly independent of the brain.

And these arms can continue reacting to stimuli even after they are no longer connected to the main brain; in fact, they remain responsive even after the octopus has been euthanized and the arms severed.

In one experiment, researchers chopped off euthanized octopuses’ tentacles, chilled them in water for an hour, and then still managed to get a split-second response when they probed the severed limbs. Other research found that, when encountering a piece of food, a severed limb will snatch it up and try to move it in the direction of a phantom octopus mouth.

If an octopus’ arm is cut off without the poor guy being euthanized, it’s no sweat for the cephalopod. While cut-off limbs do not regrow a new octopus, à la starfish, the octopus can regenerate tentacles with a far superior quality than, say, a lizard’s replacement tail, Harmon writes.

To do this, octopus use a protein called protein acetylcholinesterase, or AChE. Humans have this protein, too, but our store of the molecule is much less active than an octopus’. Harmon describes what happens when an octopus loses its leg:

Within three days, some cascade of chemical signals cued the formation of a “knob,” covered with undifferentiated cells, where the cut had been made. And further molecular signals were responsible for the “hook-like structure” that was visible at the end of the arm in the second week. Around that time, a mass of stem cells and a hefty amount of blood vessels have arrived at the site. Yet by day 28, these features disappeared. And for the next hundred days or so, the arm tip grew back in to resemble the original one.

AChE rose, peaked and dipped throughout this process, conducting a regrowth orchestra of tissues, nerves and structures until the arm was good as new. The ultimate hope, of course, is to harness the AChE trick for human limb regeneration, although that’s still a distant vision. On the other hand, we probably don’t want to start implanting neurons in our arms: imagine a severed human hand crawling across the floor, creating a real-life Addams Family moment.

More from Smithsonian.com:

Fish Mimics Fish-Mimicking Octopus 
How an Octopus Eats 

Rachel Nuwer | | READ MORE

Rachel Nuwer is a freelance science writer based in Brooklyn.