Why Octopuses Don't Tie Themselves in Knots

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Octopuses look like a tangle of squishy arms, and yet they never tie their limbs in a knot, finds a new study that determined octopus skin repels itself.

The discovery, published in the latest issue of Current Biology, is helping researchers to design soft robots, such as for surgical use, that can reshape their bodies without becoming a jumbled mess.

Photos: Tentacled Who’s Who: Bizarre and Extreme Cephalopods

It also solves a mystery about octopuses, whose brains appear to be unaware of what their two legs and six arms are doing. A chemical produced by octopus skin temporarily prevents octopus suckers from sucking.

"We were surprised that nobody before us had noticed this very robust and easy-to-detect phenomena," co-author Guy Levy of the Hebrew University of Jerusalem, said in a press release. "We were entirely surprised by the brilliant and simple solution of the octopus to this potentially very complicated problem."

We humans don't have such problems because our rigid skeletons limit the number of possibilities of where our arms and legs could be.

"Our motor control system is based on a rather fixed representation of the motor and sensory systems in the brain in a formant of maps that have body part coordinates," explained co-author Binyamin Hochner.

He continued, "It is hard to envisage similar mechanisms to function in the octopus brain because its very long and flexible arms have an infinite number of degrees of freedom. Therefore, using such maps would have been tremendously difficult for the octopus, and maybe even impossible."

As a demonstration of that freedom, check out this video that shows just how amazingly flexible these marine animals can be:

For the study, Hochner and his colleagues observed the behavior of amputated octopus arms, which remain very active for an hour after separation. (Octopuses can grow their arms back, but doing so is energetically costly.)

Those observations showed that the arms never grabbed octopus skin, though they would grab a skinned octopus arm. The octopus arms didn’t grab Petri dishes covered with octopus skin either, and they attached to dishes covered with octopus skin extract with much less force than they otherwise would.

"The results so far show, and for the first time, that the skin of the octopus prevents octopus arms from attaching to each other or to themselves in a reflexive manner," the researchers wrote. "The drastic reduction in the response to the skin crude extract suggests that a specific chemical signal in the skin mediates the inhibition of sucker grabbing."

Video: Flying Man and Escape Artist Octopus!

They haven't yet identified that particular chemical, but when they do, it could lead to a new and formidable class of soft robots. If you watched the video here of the octopus, you can imagine some of the possibilities, of robots going in and out of very precarious situations and bouncing back to their original shape good as new.

"Soft robots have advantages that they can reshape their body," co-author Nir Nesher explained. "This is especially advantageous in unfamiliar environments with many obstacles that can be bypassed only by flexible manipulators, such as the internal human body environment."

The researchers shared their findings with European Commission project STIFF-FLOP, which is working on developing a flexible surgical manipulator in the shape of an octopus arm.

Image: Octopus. Credit: Wikimedia Commons

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