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Science explains why octopus arms don't stick together

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Amputated octopus arms can't grab octopus skin, and that might be great for robotics

Joe Parks / Flickr

Octopus arms can grab onto just about any smooth surface with ease and, for the most part, they do so without communicating their location to the brain. This ability has turned them into darlings of the robotics industry, which has made numerous attempts to reproduce their underwater grasping abilities. Yet, despite all the work that has gone into deciphering the mechanics of octopus suckers, researchers have never asked one seemingly glaring question: how do octopuses avoid getting their suckers stuck to their own skin if they have no idea where their arms are most of the time?

According to a new study published today in Current Biology, the answer is chemical. Through a series of experiments, researchers were able to figure out that octopuses produce molecules in their skin that prevent their arms from getting tangled. Moreover, under certain conditions, these animals are able to stop those molecules from doing their thing in order to grasp other octopuses. "Everybody knew the lack of knowledge in octopus arms, but nobody wanted to investigate this," says Guy Levy, a neuroscientist at the Hebrew University of Jerusalem and a co-author of the study. "Now we know that they have a built-in mechanism that prevents them from grabbing octopus skin."

"The brain wants to grasp the amputated arm, but the arm is refusing."

What is most surprising, however, is that the "off-switch" that allows the animals to grasp the skin of other octopuses doesn't work perfectly. In fact, it's pretty glitchy. "In some cases, when we submitted an amputated arm, the octopus would grab it like any other food item, holding it tightly with a web of skin between its arms," Levy says. "But sometimes the octopus would get close to the arm, dance around it from side to side for a long time — sometime even tens of minutes." This, Levy says, indicates that there might be an internal conflict in the octopus' nervous system, where the arm — which has its own decentralized nervous system — and the brain are sending different, highly contradictory signals. "The brain wants to grasp the amputated arm," Levy says, "but the arm is refusing."

To study this phenomenon, the researchers came up with a number of novel experiments, most of which involved watching amputated octopus arms grab various objects. "An octopus arm is lively for more than an hour after amputation," Levy says, and they retain the ability to attach to "just about anything" during that period. But even when separated from the rest of its body, octopus arms are still unable to grasp fresh octopus skin — whether it's attached to an octopus or not. "We thought that the reason might be electrical," Levy says, but the amputated arms had no trouble grabbing onto skinned octopus arms, so an electrical mechanism seemed unlikely.

In another experiment, the researchers demonstrated that the mechanism wasn't texture or electricity related because the amputated arms couldn't grab "reconstructed skin" that had been broken down to its constituent molecules and embedded in a gel. Thus, only one possibility remained: a chemical one.

The "program" stays on after the arm is amputated

Unfortunately, there's still a lot that the researchers don't know. "We do not know which molecules are involved," Levy says, "but we do know that molecules in the skin are sensed in the suckers and this inhibits the attachment behavior." This, the researchers think, is a "built-in program" that stays on after the arms are amputated. When an arm is still attached to its owner, however, "the brain can decide to cancel the program and force the arm to grab the skin."

Except that sometimes that doesn't work very well, and octopuses end up "dancing" around while their nervous systems figure out how to deal with conflicting signals. "The finding itself is very interesting, I think," says Cecilia Laschi, a biorobotics professor at The Scuola Superiore Sant'Anna in Italy who did not participate in this study, but has worked on building an octopus robot. "It's something that I think nobody investigated before, but in fact the finding is really impressive." Laschi thinks that reproducing this exact mechanism in soft robots might prove impossible, but researchers could "use the principle" to come up with "some other mechanism for inhibition of the attachment of the suckers."

a surgical tool that avoids certain objects

This is something that Levy and his team are already thinking about. Along with a group of European researchers currently running a soft-robotics project called STIFF-FLOP, Levy would like to build a surgical tool that imitates the octopus's ability to preferentially avoid grasping certain objects. "We are aiming at building a surgical soft-manipulator that might be able to scroll inside the human body while avoiding interactions between arms and parts of the human environment that aren't involved in its tasks — like intestinal walls."

This sort of tool is still a long way off, Levy cautions. Right now, the researchers are hoping to figure out which molecules are at play, and whether other species of octopuses and cephalopods use the same mechanism. "We are finding more and more surprising information about this animal all the time," Levy says. Because of their unique structure, he says, octopuses developed completely new mechanisms over time, so it will take a while before we understand how they all work. "It is really a special animal."