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."
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. 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."