The Colours of Octopus: Using Spectral Data to Measure Octopus Camouflage.

No animal can so effectively camouflage in such a wide range of environments as the octopus. Thanks to their highly malleable skin, they are capable of adapting their body patterns to the brightness and texture of their immediate environment, and they often seemingly match the colour of background objects. However, octopuses are colour-blind as their eyes have only one type of visual pigment. Therefore, chromatophores in their skin are likely to respond to changes in brightness, not chromaticity. To determine whether octopuses actually match background colours, we used a SpectraScan® PR-655 spectroradiometer to measure the reflectance spectra of Octopus tetricus skin in captivity. The spectra were compared with those of green algae, brown algae, and sponges-all of these being colourful objects commonly found in the octopus's natural environment. Even though we show that octopuses change both lightness and chromaticity, allowing them to potentially camouflage in a wide range of backgrounds in an effective manner, the overall octopus colours did not reach the same level of saturation compared to some background objects. Spectra were then modelled under the visual systems of four potential octopus predators: one dichromatic fish (Heller's barracuda), two trichromatic fish (blue-spotted stingray and two-spotted red snapper), and one tetrachromatic bird (wedge-tailed shearwater). We show that octopuses are able to match certain background colours for some visual systems. How a colour-blind animal is capable of colour-matching is still unknown.

Angular dependence of polarisation contrast sensitivity in octopus.

Cephalopod photoreceptors are polarisation-sensitive, giving them an ability to discriminate between lights of different angle and degree of polarisation. While colour vision is achieved by comparison of signals of photoreceptors tuned to different parts of light spectra, polarisation vision is achieved by comparison of signals of photoreceptors tuned to different orientations of e-vector. Therefore, from a theoretical point of view, polarisation vision is similar to colour vision. In particular, detection of polarised light against an unpolarised background is analogous to detection of chromatic light against grey. The dependence of polarisation contrast sensitivity on the angle of polarisation can be theoretically predicted using a receptor noise limited model in much the same way as it has been done for predicting the shape of the increment threshold spectral sensitivity in animals with colour vision. Here we report angular dependence of polarisation contrast sensitivity in octopus (O. tetricus, Gould 1852) and compare the theoretical predictions of polarisation contrast with the experimental results. Polarisation gratings were generated using LCD screens with removed polarisers and the orientation of polarisation was changed by rotating the screen. Reaction to the stimulus was recorded using a fixation reflex. We show that, in agreement with the theoretical predictions, the maximum contrast sensitivity is achieved at horizontal and vertical orientations of polarisation. Our results demonstrate that the dependence of polarisation contrast sensitivity on the angle of polarisation can be analysed in the same way as the dependence of colour thresholds on wavelength of monochromatic light added to a grey background.

Spatial Contrast Sensitivity to Polarization and Luminance in Octopus.

While color vision is achieved by comparison of signals of photoreceptors tuned to different parts of light spectra, polarization vision is achieved by comparison of signals of photoreceptors tuned to different orientations of the electric field component of visible light. Therefore, it has been suggested that polarization vision is similar to color vision. In most animals that have color vision, the shape of luminance contrast sensitivity curve differs from the shape of chromatic contrast sensitivity curve. While luminance contrast sensitivity typically decreases at low spatial frequency due to lateral inhibition, chromatic contrast sensitivity generally remains high at low spatial frequency. To find out if the processing of polarization signals is similar to the processing of chromatic signals, we measured the polarization and luminance contrast sensitivity dependence in a color-blind animal with well-developed polarization vision, Octopus tetricus. We demonstrate that, in Octopus tetricus, both luminance and polarization contrast sensitivity decrease at low spatial frequency and peak at the same spatial frequency (0.3 cpd). These results suggest that, in octopus, polarization and luminance signals are processed via similar pathways.

Contrast sensitivity and behavioural evidence for lateral inhibition in octopus.

Behavioural contrast sensitivity in Octopus tetricus was measured in the range of 0.05-12 cycles per degree (cpd) using a fixation reflex. We show that the contrast sensitivity reaches its maximum (between 1 and 4%) at 0.3 cpd, and decreases to approximately half of the maximum value at the lowest spatial frequency. Reduction of sensitivity at low spatial frequency is a signature of lateral inhibition in visual systems. In vertebrates and insects, lateral inhibition helps to overcome the bottleneck of encoding information into spikes. In octopus, photoreceptors generate spikes themselves and are directly connected to the brain through their axons. Therefore, the neural processing occurring in the octopus brain cannot help overcome the bottleneck of encoding information into spikes. We conclude that, in octopus, either the lateral inhibition occurs in the brain after information has been encoded into spikes, or photoreceptors inhibit each other. This is the first time behavioural contrast sensitivity has been measured in a cephalopod.