Common octopus. Credit: H. Zell CC BY-SA 3.0)
Common octopus. Credit: H. Zell CC BY-SA 3.0)

Why Octopus Arms Don’t Get Tangled

If you cut off an octopus’s arm, the severed limb will still move about for at least an hour. That’s because each arm has its own control system—a network of around 400,000 neurons that can guide its movements without any command from the creature’s brain.

The hundreds of suckers along each arm can also behave independently. If a sucker touches an object, it will change its shape to form a tight seal, and contract its muscles to create a powerful suction. It grabs and sucks, by reflex.

This setup allows the octopus to control its astonishing appendages without overly taxing its brain. Your arm has a small number of joints and can bend in a limited number of ways. But an octopus’s arm can create as many joints as it wants, in any direction, anywhere along its length. It can also extend, contract, and reshape itself. To control such infinitely flexible limbs, it needs to outsource control to the limbs themselves.

But what happens if one arm brushes past another? If the suckers grab objects on reflex, why aren’t octopuses constantly grabbing themselves by mistake?

To find out, octopus arm expert Benny Hochner teamed up with octopus sucker expert Frank Grasso.“Octopus suckers are undervalued in terms of their complexity,” says Grasso. “I’m one of their proponents. They’re really exquisite manipulation devices.”

Together with Nir Nesher and Guy Levy, the duo noticed that the suckers on a freshly amputated arm will never attach to another arm. Sure, they’ll grab skinned parts of an amputated arm or the bare flesh at the point of amputation, but not the arm itself. They’ll grab Petri dishes, but not those that are covered with octopus skin.

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Common octopus. Credit: Pseudopanax.

Octopuses clearly have some kind of sucker-proof coating on their own skin.  The team confirmed this idea by extracting chemicals from the skins of both fishes and octopuses, and applying these cocktails onto Petri dishes. They found that the octopus extract could block a sucker’s grabbing reflex but the fish extract could not.

“We all knew that octopuses are very dependent on chemical sensing but we haven’t done much research on this,” says Jennifer Mather from the University of Lethbridge, who studies octopus behaviour. “This paper will probably kick start it.”

Whatever the mystery chemical, it’s clear that octopuses can override its influence. The team showed that that living animals will occasionally grab amputated arms, even by the skin. Their brains can veto the reflexes of their suckers.

They can even tell if an amputated arm belonged to them or to another octopus. If they sensed another individual’s severed arm, they would often explore it, grab it, and hold it in their beaks in an unusual posture that the team called “spaghetti holding”. (Common octopuses will cannibalise their own kind, so a floating arm is fair game.) But when they sense their own severed limbs, they typically avoided it, and only rarely treated it like food.

“This gives us some idea of how octopuses might generate a sense of self—not by vision, which would be hopeless given their changeable appearance, but by chemical cues,” says Mather.

The octopus’s self-avoiding arms are a great example of embodied cognition—the idea that an animal’s body can influence its behaviour independently of its brain. As Andrew Wilson and Sabrina Golonka explain, “the brain is not the sole resource we have available to us to solve problems. Our bodies… do much of the work required to achieve our goals.”

The octopus… well… embodies this idea. Its brain governs many of its decisions and exerts control upon its arms, but the arms can do their own thing, including getting out of each others’ way. The animal doesn’t need to know the location of each of its arms to avoid embarrassing entanglements. It can let its arms do the work of evading each other.

This concept might be useful for designing robots. A typical robot, like Honda’s ASIMO, relies on top-down programs that control his every action. He can pull off pre-programmed feats like dancing or running, but he trips over even minor obstacles. He’s inflexible and inefficient. By contrast, Boston Dynamics’ Big Dog relies on embodied cognition. His springy legs are designed to react to rough terrain without needing new instructions from his central processor. (Thanks again to Wilson and Golonka for the examples.)

By studying the arms of octopuses, scientists may one day be able to design soft versions of Big Dog, pairing its flexible movements with an equally flexible chassis. Big Octopus, perhaps.

Reference: Nesher, Levy, Grasso & Hochner. 2014. Self-Recognition Mechanism between Skin and Suckers Prevents Octopus Arms from Interfering with Each Other. Current Biology. http://dx.doi.org/10.1016/j.cub.2014.04.024

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