Octopuses' Sensitivity to Light Generates Defensive Behaviour
Octopuses are well-known to be complex and highly intelligent creatures, and new research suggests they are even more talented than previously understood, with the revelation that octopuses are able to 'see' light through the tips of their arms.
The phenomenon was first observed by one of the study's co-authors Itamar Katz, of the Faculty of Marine Sciences at Ruppin Academic Center in Israel, while he was researching the mechanism behind which octopuses are able to change their skin colour.
Octopus skin – like other cephalopods such as squid and cuttlefish – contains colour-changing cells known as chromatophores. These are what give octopuses the ability to blend into their surroundings, but they also appear to be sensitive to light, changing size and shape when illuminated.
While studying the 'eye-independent, light-activated chromatophore expansion' (LACE), Katz noticed that octopuses exhibited a 'reflex-like' action when a light beam was shone on their arm-tips, causing the animal to withdraw its arm and fold it in towards its body.
'We were using a very strong flashlight and when we illuminated the tip of the arm, it would always pull away. It was very surprising,' said Tal Shomrat, one of the study's co-authors. 'We shifted our experiment to explore this behaviour after we found out that nobody had described it before.'
During the experiment, an octopus was placed in a tank with an opaque covering and trained to retrieve a piece of food by pushing its arm through a small hole in the cover. When the researchers shone a powerful torch over the arm, the octopus would rapidly withdraw the appendage, even though it could not have seen the light through its eyes.
The researchers also measured the water temperature to show that the reaction was not to a sudden increase in heat. 'We often feel the heat from intense light, but for the octopus, this isn't the case,' said co-author Nir Neshir. 'In our experiments, we checked for changes in temperature and there weren't any. The effect is from pure light.'
The phenomenon, known as 'phototaxis', has previously been described in some species of plankton, jellyfish and insects, but this is the first time it has been observed in cephalopods. The reaction is thought to be a defence mechanism to prevent the tips of the octopus's arms from being mistaken for worm-like creatures and attacked by predators.
While their arms may be incredibly sensitive, octopuses have very poor 'proprioception' – they are unable to sense exactly where their body and arms might be at any given time. 'In the octopus, you have no bones and no joints, and every point in its arm can go to every direction that you can think about,' said Nesher. 'So even one arm, it’s something like endless degrees of freedom.'
The researchers concluded that the phototactic response in octopuses compensates for their poor proprioception by making sure that their arms are protected in an illuminated area, 'without the need to be aware of their state.' The report also suggests that the phenomenon that has been observed in octopus for many years – but nobody really noticed why they were doing it.
'The behaviour is so robust and obvious, it is interesting that nobody had described it before,' said Shomrat. 'Now, our next steps are to investigate the evolution and purpose of the behaviour.'
'Feel the light: sight-independent negative phototactic response in octopus arms' by Itamar Katz, Tal Shomrat and Nir Nesher is published in the Journal of Experimental Biology at jeb.biologists.org/content/224/5/jeb237529