Highly trained dancers and athletes are exquisitely attuned to the location of their own limbs. The precision of trained professionals is a dimension beyond most of our vaguely placed manoeuvres. But we only have four limbs to keep track of, so spare a thought for octopuses. With eight infinitely manoeuvrable limbs, how do these cephalopods keep track of them all? The simple answer is, they don't. ‘Such a system would require a huge computational load’, says Nir Nesher from the Ruppin Academic Center, Israel. Yet octopuses that conceal themselves by day in shady nooks need to be sure that all of their limbs are safely secured out of sight to stop passing fish and crustaceans giving them a nibble. So, when Itamar Katz noticed that a common octopus jerked its tentacles back when he shone light on the tips of their arms during an undergraduate experiment, Tal Shomrat and Nesher were surprised. ‘That was how we first noticed this exceptional behaviour. It was a big surprise for us that no one reported it before’, says Shamrat. Could this simple strategy help octopuses keep the tips of their tentacles safe when they emerge inadvertently from the safety of secluded crevices?

But first the team needed to confirm that the octopuses’ lithe limbs actually ‘feel’ light. Impressively, the octopuses all snatched their limbs away when the team pointed a bright torch at the tip. And when they shone light on a tasty chunk of mackerel that the octopus had to search for from behind a screen, the hunting limb almost always avoided the illuminated treat, only returning to retrieve it when the scientists turned off the spotlight. The team then tested whether the nerve or muscles in the flexible appendage were essential for triggering the octopuses’ light-sensitive reflex and found that both were, suggesting that messages were passing to the brain through the muscle itself, as well as the tentacle nerve, to trigger the response. In addition, anaesthetised octopuses that had lost the connection between the limb and their brain no longer snatched their arm back from a light encounter, although they still reacted when the trio gently pinched the limb. ‘The results point toward a reflex arc-like control configuration that involves the brain for the phototactic response’, says Nesher.

It seems that the reflex-like reaction could help octopuses keep the tips of their arms safe from famished fish without having to keep track of their precise location and the team is now eager to discover how feeling light triggers octopuses to pull in their tentacles for safety.

Feel the light: sight-independent negative phototactic response in octopus arms
J. Exp. Biol.