With hundreds of suckers on each of their eight gangly arms, it’s a wonder how octopuses don’t tie themselves up in knots all the time. A new study reveals how they manage this feat: Chemicals produced by their skin temporarily prevents their suckers from sucking.
Their bowl-shaped suckers are made of thick muscles, which help them stick to just about anything. And it’s a reflex. Experiments have shown that octopuses don’t know where their arms are exactly -- unlike humans and our rigid skeletons. Our brains have a fixed representation of our motor and sensory systems, like a map with body part coordinates, according to Binyamin Hochner from Hebrew University of Jerusalem. "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," Hochner explains in a news release. "Therefore, using such maps would have been tremendously difficult for the octopus, and maybe even impossible."
Hochner and his Octopus Group observed the behavior of amputated octopus arms. (Severed arms can stay very active for an hour afterwards, and these amazing animals can also regrow their limbs after they, say, fling it away to distract predators.) The team noticed that the arms never grabbed octopus skin, though they would grab an octopus arm that’s been skinned. The dismembered arms also didn’t grab Petri dishes covered by octopus skin, and if the dishes were covered by octopus skin extract (a gel made form skin soaked in an alkane), the arms attached with much less force than they normally would; they preferred latching on to any of the plastic that’s exposed.
"We were surprised that nobody before us had noticed this very robust and easy-to-detect phenomena," study author Guy Levy of Hebrew University says. "We were entirely surprised by the brilliant and simple solution of the octopus to this potentially very complicated problem."
Live octopuses, however, can override that automatic mechanism, if it’s convenient. When presented with amputated arms, Octopus vulgaris will sometimes grab at them, especially if they weren’t formerly one of their own -- a clear indication of self-recognition. Sometimes they’d grabbed the arms at the site where flesh was exposed, bring them to their mouths, and hold them with their beaks for a while -- researchers call this behavior “spaghetti holding.”
“About two-thirds of the octopus’ nervous system is in the arms. The local nervous system of the arms locally analyzes sensory information and sends ‘conclusions’ to the brain,” Levy tells The Scientist. That’s why the suckers, under local control, don't grab the amputated arm, whereas the mouth makes contact. You can watch an amputated arm move in this video. There’s also footage of spaghetti holding.
The team hasn’t isolated the active agent underlying the self-avoidance behavior, though once someone figures it out, the strategy would be very useful for soft surgical robots to avoid obstacles inside the human body.
The work was published in Current Biology this week.
Image: Joe Parks via Flickr CC BY 2.0
Video: Current Biology, Nesher et al. via The Scientist