Abstract
ABSTRACTPolarization vision is widespread in nature, mainly among invertebrates, and is used for a range of tasks including navigation, habitat localization and communication. In marine environments, some species such as those from the Crustacea and Cephalopoda that are principally monochromatic, have evolved to use this adaptation to discriminate objects across the whole visual field, an ability similar to our own use of colour vision. The performance of these polarization vision systems varies, and the few cephalopod species tested so far have notably acute thresholds of discrimination. However, most studies to date have used artificial sources of polarized light that produce levels of polarization much higher than found in nature. In this study, the ability of octopuses to detect polarization contrasts varying in angle of polarization (AoP) was investigated over a range of different degrees of linear polarization (DoLP) to better judge their visual ability in more ecologically relevant conditions. The ‘just-noticeable-differences’ (JND) of AoP contrasts varied consistently with DoLP. These JND thresholds could be largely explained by their ‘polarization distance’, a neurophysical model that effectively calculates the level of activity in opposing horizontally and vertically oriented polarization channels in the cephalopod visual system. Imaging polarimetry from the animals’ natural environment was then used to illustrate the functional advantage that these polarization thresholds may confer in behaviourally relevant contexts.
Highlights
Polarization vision is widespread in nature (Roberts et al, 2011), mainly among the invertebrates, where it contributes to a variety of behavioural tasks including navigation (Wehner, 1976), habitat localization (Schwind, 1991) and communication (Chiou et al, 2008)
We examined the threshold of detection of polarization contrasts in two species of octopus (Abdopus aculeatus and Octopus cyanea) by varying both angle of polarization (AoP) and degree of linear polarization (DoLP) using a modified liquid crystal display (LCD) to deliver dynamic polarization stimuli (Basnak et al, 2018; Glantz and Schroeter, 2007; How et al, 2012; Pignatelli et al, 2011; Temple et al, 2015)
Octopuses responded to looming stimuli varying in polarization contrast alone and were sensitive to very small ΔAoP, when the degrees of linear polarization (DoLP) was high
Summary
Polarization vision is widespread in nature (Roberts et al, 2011), mainly among the invertebrates, where it contributes to a variety of behavioural tasks including navigation (Wehner, 1976), habitat localization (Schwind, 1991) and communication (Chiou et al, 2008). In terrestrial environments, this sensory capacity is best understood in the dorsal rim area of the eye, which is directed towards the sky to enable the detection of the pattern of celestial polarization (Wehner, 1976). Several species of crustacean and cephalopod incorporate optical structures in their cuticle or skin, which produce strongly polarized signals for communication (Chiou et al, 2008; Marshall et al, 2019; Shashar et al, 1996)
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