On the Edge: The Incredible Regeneration of an Octopus's Arms

In the 2020 Academy Award-nominated documentary My Octopus Teacher, filmmaker Craig Foster forms an extraordinary bond with a wild octopus in the kelp forests off South Africa. In one poignant moment—minor spoiler ahead—the octopus endures a brutal injury when a shark tears one of its limbs off.

Gradually, almost miraculously, the cephalopod's severed arm starts to regenerate. This feat is particularly astounding because octopuses house two-thirds of their neurons in their arms, with nerve cords in each limb functioning similarly to the spinal cord in humans. So how does it cope when it loses not just a limb, but a portion of its nervous system?

The Challenge of Disarming an Octopus

To understand how an octopus regenerates its arm, it’s important to first grasp what an octopus’s limb actually is. Lacking a skeleton, an arm consists of a nerve cord and three muscle bundles—transverse, longitudinal, and oblique. Limbs can be damaged through injury, attacks from predators, during mating, or even through self-cannibalism, where the octopus eats its own arm. While scientists are uncertain about the cause, it’s believed to be linked to a nervous system disorder or a survival strategy against predators.

The loss of an arm can be a painful experience for an octopus. A 2021 study published in iScience showed that octopuses may indeed feel pain. Researchers found that the creatures avoided areas of their tanks where they had been subjected to painful stinging injections. Interestingly, they gravitated toward zones where pain relief treatments had been administered.

When an octopus loses a limb, it undergoes a process known as morphallaxis, where the damaged tissue is reorganized to enable regrowth. Everything from nerve bundles to suckers regenerates as cells at the injury site fragment into multiple particles, kickstarting the development of new muscles and nerves.

Caroline Albertin, Ph.D., an octopus expert from the University of Chicago Marine Biological Laboratory, explains that this type of regeneration replaces the typical wound-healing process found in other species.

“Typically, when an octopus loses an arm, instead of forming a scab or scar like humans do, the wound is covered by a layer of cells called the epithelium,” Albertin explains to Mytour. “Beneath the epithelium, undifferentiated cells called a blastema begin to accumulate. These cells divide to form new cells, gradually pushing the blastema outward. Within a few days, we start seeing early structures, like tiny suckers, emerging from the regenerating arm.”

It takes around three days for cells to cover the injury site, adopting a hook-like form. Within two weeks, stem cells and blood vessels flood the area. In about 130 days, the octopus will have grown a fully functional new arm.

A particular protein, acetylcholinesterase (AChE), may play a crucial role in this process. A 2013 study published in the Journal of Experimental Marine Biology and Ecology examined the biochemical changes in eight octopuses after a small part of their arms was amputated. Starting around the third week post-surgery, researchers noticed a significant increase in AChE levels as new suckers and chromatophores (responsible for the octopus's color-changing ability) began to form. By the time the arm had fully regenerated, AChE levels returned to normal.

Once the arm has fully regenerated, the octopus seems to experience no negative consequences. “Given sufficient time, a regenerated arm can function just as effectively as the original,” says Albertin. “It can grow to full size.” However, whether there are any changes in its biomechanical properties or strength remains an unanswered question. “I’m not sure if anyone has studied if there are any deficits in strength or movement,” she adds.

The Octopus Arms Race

So, what happens to the severed arm of an octopus? Can it, like the disembodied hand in 1987's Evil Dead II, develop a life of its own after being detached from its owner? After all, it contains neurons.

The answer is: Sort of. In another 2013 study published in the Journal of Experimental Marine Biology and Ecology, researchers removed arms from 10 octopuses shortly after their death. An hour later, the team stimulated the severed limbs by pinching or applying tap water and acetic acid. The limbs reacted by flinching to each stimulus. However, plain seawater or gentle pressure did not provoke any response.

The regenerative abilities of octopuses continue to captivate scientists. In November 2020, a nine-armed octopus was found in Shizugawa Bay, Japan. One of its arms appeared to have split from another, suggesting the possibility that it had regenerated two limbs.

Octopuses likely have the capacity to regenerate several limbs at once. “We’ve observed instances where multiple arms regenerate simultaneously,” says Albertin.

If you're curious, humans also possess some limited regenerative abilities. However, they're confined to the tips of our fingers, and this ability is most apparent in children under the age of 10.