WHILE some may mislay their keys on a difficult day, octopuses can lose their arms. Stuck in a tight spot, the creatures leave behind arms causing more trouble than they’re worth. Controlling the movements of these flexible appendages is difficult—octopuses use rehearsed motions as one means of keeping them in order.
Another has been probed by Binyamin Hochner from the Hebrew University of Jerusalem and his team in a study just published in Current Biology. The findings show for the first time that each arm’s motor programme, to a large extent contained within its own nervous system, is regulated using chemical self-recognition mechanisms within the appendage. Latest updates
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Octopus arms know what they should, and should not, pick up using their 300 suckers—octopus skin itself appears in many cases taboo (explaining why these creatures do not stick to themselves). But the team also discovered that the higher centres of a common cephalopod’s brain can override the arm’s sensory inclinations (provided, of course, it is still attached to an individual).
Freshly amputated octopus arms remain highly active for more than an hour after removal from their owner. Over 30 trials, researchers saw that suckers on such arms never grasped themselves, other amputated arms, or petri dishes covered with octopus skin. Skinned arms, the flesh at the site of the amputation and the plastic of the dishes were another story, however, suggesting that something within octopus skin inhibits suckers’ attachment reflex.
Suckers also grasp different items with varying levels of force. Clasping a piece of fish, for example, used 67% of the effort needed to grab a plastic dish, while holding on to boiled and peeled shrimps took just 16%. Sensory mechanisms within suckers know what they are dealing with.
To discover whether it was a physical property of the skin, such as its texture, that repelled suckers, or whether other molecules in the skin itself were responsible, scientists studied the response of amputated arms to petri dishes coated in a gel soaked with octopus skin crude extract. Amputated arms avoided grasping them meaning that molecules within the skin itself trigger its effect on suckers. Guy Levy, a co-author alongside Nir Nesher, admits that as yet the researchers are unaware of the molecules’ exact composition, however.
Arms attached to octopuses can behave differently from those without a master. The octopus vulgaris, known to be cannibalistic, will pick up items covered in octopus skin—the creature’s brain can override the reflexive inhibition of a particular arm’s suckers.
But there is a slight catch. In 16 out of 17 trials octopuses picked up an amputated arm that did not belong to them, and in 72% of further tests treated them as food. But when served up their own lost arm, suckers attached in less than 40% of cases and, separately, octopuses tasted their own juiciness just 28% of the time. When amputated arms were skinned, however, they were enjoyed as typical food by owners and others alike.
The team wish to find out which of the 40m tactile and chemical receptors on octopus arms (concentrated within suckers) can detect octopus skin and, more specifically, how they interact with the higher centres of an octopus’s brain to differentiate between their own skin and that of other creatures.
These self-recognition and self-avoidance mechanisms in octopuses may help robot designers reckons Dr Levy. The cephalopods’ motor control systems reveal much about the possibilities for developing adaptive units that use embodied organisation, a set-up where the autonomous nervous system of body parts, the control centre of the brain and the stimulae within the environment interact, as seen in octopuses. For now, however, the study fully explains why octopuses do not tie themselves into self-sticking knots (just in case you have ever contorted your own brain trying to work it out).