Behavioural evidence of spectral opponent processing in the visual system of stomatopod crustaceans.

Stomatopods, commonly known as mantis shrimps, possess intricate colour vision with up to 12 photoreceptor classes arranged in four specialised ommatidia rows (rows 1-4 in the midband region of the eye) for colour perception. Whereas 2-4 spectral sensitivities suffice for most visual systems, the function and mechanism behind stomatopods' 12-channel colour vision remains unclear. Previous anatomical and behavioural studies have suggested that binning and opponent processing mechanisms may coexist in stomatopod colour vision. However, direct evidence of colour opponency has been lacking. We hypothesised that if colour opponency exists in stomatopod vision, they would be able to distinguish colour from grey under coloured illumination. Conversely, if only the binning system is used, they would not. By examining the colour vision of the stomatopod Haptosquilla trispinosa with modified von Frisch grey card experiments, we found that they can differentiate between colour and grey under various coloured illuminations. Our results provide the first direct behavioural evidence of spectral opponency in stomatopods, suggesting that they use a hybrid colour processing system combining opponent and binning mechanisms for colour vision. This study advances our understanding of the complex visual system in stomatopods and highlights the importance of further research into the processing mechanisms, function and evolution of their unique visual system.