In the 2020 Oscar-nominated documentary My Octopus Teacher, filmmaker Craig Foster strikes up a fascinating friendship with a wild octopus in a South Africa kelp forest. In one scene—mild spoiler here—the mollusk suffers what appears to be a grievous injury when a shark rips off one of its arms.

Slowly, seemingly miraculously, the cephalopod’s appendage begins to grow back. This is especially impressive considering that octopuses host two-thirds of their neurons in their arms, thanks to nerve cords in each that act much like a spinal cord does in humans. So what happens when they lose not only a limb, but also part of their nervous system?

Disarming an Octopus

First, it helps to understand what an octopus’s appendage really is. They have no skeletal structure; instead, the arm consists of a nerve cord and three muscle bundles—transverse, longitudinal, and oblique. Limb damage can come from injury, predators, or even copulation. It can also come from a kind of self-cannibalism, where the octopus chows down on its own limb. (Scientists aren’t sure why that happens, though it could be a nervous system disease. They may also do it to escape a predator.)

Losing a limb is a potentially painful event for the octopus. A 2021 study in iScience demonstrated that octopuses may feel pain because they avoided parts of their tanks where researchers had given them stinging injections. The animals preferred the areas where they had received pain-relieving treatments.

When an octopus loses an arm, everything from nerve bundles to suckers are regenerated in a process called morphallaxis, where existing tissue is rearranged to allow for new tissue to grow. Cells at the wound site fragment into multiple particles to promote new muscles and nerves.

According to Caroline Albertin, Ph.D., an octopus expert at the University of Chicago Marine Biological Laboratory, the regeneration takes the place of typical wound closure in other species.

“In general, when an octopus loses an arm, instead of forming a scab or a scar, like what happens when we get a wound, we see that the wound gets covered by a layer of cells called the epithelium,” Albertin tells Mental Floss. “And under the epithelial cells, we start to see an accumulation of undifferentiated cells called a blastema. These cells divide to make more cells, pushing the blastema out. Within a couple of days, we see some differentiated structures—like little suckers—sticking out of the regenerating part of the arm.”

It takes about three days for cells to cover the amputation site and take on a hook-like shape. Within two weeks, stem cells and blood vessels pour in. In approximately 130 days, the octopus will have gained another fully-functioning arm.

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A specific protein, acetylcholinesterase, or AChE, may be providing a lot of assistance. A 2013 study published in the Journal of Experimental Marine Biology and Ecology looked at the biochemical processes of eight octopuses following the amputation of a small part of the tip of their arms. Beginning roughly the third week following surgery, researchers saw a dramatic rise in the animals' AChE levels just as new suckers and chromatophores (which give the octopus its color-changing abilities) were developing. By the time the arm was fully restored, AChE levels had dropped to baseline.

Once the arm is fully grown, it’s likely the octopus suffers no ill effects from having regenerated an entire part of its body. “Given enough time, a regenerated arm can function just like the original arm,” Albertin says. “It can reach full size.” Whether it’s impaired in any way is more of an open question. “I don’t know if anyone has looked to see if there are any deficits in its biomechanical properties or strength,” she says.

The Octopus Arms Race

That’s what happens to the octopus. But what happens to the severed arm? Can it, in the spirit of 1987’s Evil Dead II, go on to have a mind of its own after being separated from its host? It does have neurons, after all.

The answer: Kind of. In another 2013 study published in the Journal of Experimental Marine Biology and Ecology, researchers severed arms from 10 octopuses immediately post-mortem. After one hour, the scientists stimulated the disembodied limbs by pinching or applying tap water or acetic acid. All stimuli provoked the limbs into flinching. Plain seawater or mild compression didn’t incite any response.

Octopuses and their regenerative abilities continue to be a source of fascination for researchers. In November 2020, a nine-armed octopus was discovered in Shizugawa Bay in Japan. One of its arms was branched off from another, meaning it might have potentially regrown two limbs.

Octopuses can probably handle regenerating several. “We have seen examples of multiple arms regenerating at the same time,” Albertin says.

In case you're wondering, humans also have some limb-regenerating abilities. But they’re limited to the tips of our fingers, a feat displayed primarily by kids under 10 years old.

[h/t Scientific American]