Octopuses avoid limb entanglement
Octopuses have evolved an ingenious way to avoid tying themselves up in knots, despite lacking full knowledge of what their arms are doing, research has shown.
The hundreds of suckers lining the creature's tentacles will stick to almost anything - but not the octopus itself, scientists have found.
They believe a chemical signal in the cephalopod's skin prevents self-adhesion. Without it, the flexible animals would quickly wind up in a tangle.
"We were surprised that nobody before us had noticed this very robust and easy-to-detect phenomena," said researcher Dr Guy Levy, from the Hebrew University of Jerusalem in Israel.
"We were entirely surprised by the brilliant and simple solution of the octopus to this potentially very complicated problem."
The discovery could help researchers designing bio-inspired "soft" robots with tentacle-like manipulators.
Unlike animals with rigid skeletons, octopuses never know exactly where their arms are at any given moment.
Professor Binyamin Hochner, who led the Hebrew University team, said: "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 co-ordinates.
"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."
Experiments have confirmed that octopuses lack accurate knowledge about the position of their arms. Given the clingy nature of their suckers, why they did not end up tying themselves in knots was an unresolved mystery.
Tests showed that amputated octopus arms, which remain highly active for an hour after separation, never grabbed hold of octopus skin. Nor did they attach themselves to lab dishes covered with octopus skin or skin extract.
Unlike amputated arms, live octopuses seem able to override the avoidance mechanism when they need to. They will, for instance, sometimes hold on to an amputated arm, especially if it is not one of their own.
The researchers wrote in the journal Current Biology: "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 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."
The scientists have not identified the active agent, but they say it is another demonstration of octopus intelligence.
They are sharing their findings with colleagues working on the European Stiff-Flop project which aims to develop a flexible surgical manipulator in the shape of an octopus arm.
"Soft robots have advantages in that they can reshape their body," said co-author Dr Nir Nesher, also from the Hebrew University. "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."