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The hundreds of suckers on an octopus’s eight arms leech reflexively to almost anything they come into contact with — but never grasp the animal itself, even though an octopus does not always know what its arms are doing. Today, researchers reveal that the animal’s skin produces a chemical that stops the octopus’s suckers from grabbing hold of its own body parts, and getting tangled up.
“Octopus arms have a built-in mechanism that prevents the suckers from grabbing octopus skin,” says neuroscientist Guy Levy at the Hebrew University of Jerusalem, the lead author of the work, which appears today in Current Biology1. It is the first demonstration of a chemical self-recognition mechanism in motor control, and could help scientists to build better bio-inspired soft robots.
To find out just how an octopus avoids latching onto itself, Levy and his colleagues cut off an octopus’s arm and subjected it to a series of tests. (The procedure is not considered traumatic, says Levy, because octopuses occasionally lose an arm in nature and behave normally while the limb regenerates.)
The severed arms remained active for more than an hour after amputation, firmly grabbing almost any object, with three exceptions: the former host; any other live octopus; and other amputated arms. “But when we peeled the skin off an amputated arm and submitted it to another amputated arm, we were surprised to see that it grabbed the skinned arm as any other item,” says co-author Nir Nesher, also a neuroscientist at the Hebrew University.
And when the researchers presented amputated limbs to their former owners, the octopuses often behaved in an unusual way, dancing around their limb and rubbing it, but not attaching any suckers. (Intact octopuses also displayed this behaviour toward amputated arms.) When an octopus spotted the exposed flesh where the limb had been severed, it sometimes grabbed the flesh, but then held it only with its beak and only by the exposed flesh — as if it were licking its wounds. The rest of the limb was left dangling freely, untouched by the other arms.
The research also shows that octopuses are able to identify their own arms, even after amputation, says Binyamin Hochner, another neuroscientist at the Hebrew University, and the principal investigator in the study, as “they grabbed amputated arms of other octopuses as food items significantly more often than their own arms”. In many of those cases they actually ate the severed limb. Octopuses of this species, Octopus vulgaris, are known to be cannibalistic.
Octopuses' brains don’t constantly monitor the exact position and motion of all of their arms; this would be very difficult because each arm has almost infinite degrees of freedom. Instead, each arm has its own 'motor controller' — a set of neurons that can control its motion independently of the rest of the body.
But sometimes the brain does step in. The fact that octopuses did eventually grab an amputated arm, but that a severed arm would never touch an octopus, suggests a conflict between what the arm is 'programmed' to do and what the brain sometimes decides to do. “We believe that the decision of the brain is much more complex and perhaps even involves high-level cognition,” says Levy.
The findings also demonstrate that vision is not the dominant sense for the octopus, and that chemical sense and touch are vital, says psychologist Jennifer Mather of the University of Lethbridge in Alberta, Canada, who was not involved in the study. “It gives us a window into the central-peripheral coordination that makes up an octopus's motor control,” says Mather.
But the study is “just a start”, she adds. “The tests are pretty crude. We don't know anything about the chemicals themselves ... We don't understand either the type or the location of the interactions between central and peripheral control. But we can see where we should look.”
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