How Octopus Arms Bypass the Brain

Octopuses’ sucker-covered arms can act as if they contain partly independent mini brains. Each arm gathers sensory information to drive its own movements—and even those of other arms—without consulting major brain regions.

“Their arms are so mobile; they’re soft, and they can bend and twist and do all sorts of things,” says Melina Hale, a biologist at the University of Chicago. In a study in Current Biology, she and her colleagues reveal the strange connections that may facilitate these supple limbs’ decentralized coordination.

The researchers investigated the anatomy of young Octopus bimaculoides, which are the size of “a big Tic Tac,” says study lead author Adam Kuuspalu, also at Chicago. They examined the octopuses’ intramuscular nerve cords: key pieces of invertebrate anatomy that contain multiple types of neurons and contribute to whole-arm movement. In most past research, “other parts of the arm nervous system were really well described, but those are just left a mystery,” Hale says.

The researchers traced the nerve cords with a powerful microscope and found that one type—the cords closest to the suckers—not only ran the length of an arm but also extended down another arm two arms away. All eight arms show this pattern. The layout was “totally different from anything that we’d ever seen before,” says Hale, who had expected the cords to create a structure similar to the central ring formed by larger peripheral nerves.

“I think it’s as simple as saying that it’s mathematically efficient,” Kuuspalu says of the newly discovered pattern. If these connections carried sensory and motor signals, they would allow for rapid communication between relatively distant arms.

San Francisco State University biologist Robyn Crook, who was not involved in the new study, says it is interesting and relevant to her work on sensorimotor integration in octopuses, squid and cuttlefish. “It’s not clear yet how the [intramuscular nerve cords] communicate or even if they do send signals across the body over long distances,” she adds. The study authors plan to delve into these questions next.

“We can now approach our anatomical and behavioral studies a bit differently, with more focus on what any one arm is doing in concert with more distant arms in the ring,” says Roger Hanlon, a researcher at the Marine Biological Laboratory and frequent collaborator with Hale’s group. “We are in that intriguing ‘mild state of confusion’ that is simultaneously perplexing and exhilarating when unexpected discoveries are revealed.”