Extraocular photoreception has been observed in many and diverse species; now they show that in response to illumination, octopuses' arm tip reacts in a reflex-like manner, folding in and moving away from light

Feel the light – sight independent negative phototactic response in octopus' arms. Itamar Katz, Tal Shomrat, Nir Nesher. Journal of Experimental Biology Feb 3, 2021: jeb.237529 doi: 10.1242/jeb.237529

Abstract: Controlling the octopus's flexible hyper-redundant body is a challenging task. It is assumed that the octopus has poor proprioception which has driven the development of unique mechanisms for efficient body control. Here we report on such a mechanism, a phototactic response of extraocular photoreception. Extraocular photoreception has been observed in many and diverse species. Previous research on cephalopods revealed that increased illumination on their skin evokes chromatophore expansion. Recently, the mechanism was investigated and has been termed 'light-activated chromatophore expansion' (LACE). In this work we show that in response to illumination, the arm tip reacts in a reflex-like manner, folding in and moving away from the light beam. We applied a set of behavioral experiments and surgical manipulations to elucidate and characterize this phototactic response. We found that in contrast to the local activation and control of LACE, the phototactic response is mediated by the brain, although it is expressed in a reflex-like pattern. Our research results and observations led us to propose that the phototaxis is a means for protecting the arms in an instinctive manner from potential daytime predators such as fish and crabs, that could identify the worm-like tips as food. Indeed, observation of behaving octopuses revealed that the arm tips are folded-in during the daytime, while at night they are extended. Thus, the phototactic response might compensate for the octopus's poor proprioception by keeping the arms folded in illuminated areas, without the need to be aware of their state.