Polarisation vision of crustaceans

The photoreceptor design of crustaceans, often containing regular arrays of intrinsically polarisation-sensitive microvilli, has had a profound influence on the visual biology of this subphylum. The land-based arthropods (insects and arachnids) also construct photoreceptors from ordered microvilli; however while in many species polarisation sensitivity results, a general overview of these groups suggests a major difference. With notable exceptions discussed in this chapter, many crustaceans seem to have “invested” in polarisation vision more than colour vision. This may be the result of the relatively limited spectral environment found in much of the aquatic world or due to the information content in polarisation being as useful as colour. The terrestrial arthropods are generally trichromatic with specialised visual areas for polarisation-specific tasks. Crustaceans are mostly di- or monochromats and most of their visual field displays polarisation sensitivity. This chapter examines the anatomical, neurophysiological and behavioural evidence for polarisation vision in a few of the many crustacean groups. Common themes are emerging such as the possession of vertical and horizontal E-vector sensitivity. This two-channel orthogonality is carried through the neural processing of information and reflected in behavioural capability. A few groups such as the stomatopods possess both complex colour and polarisation sensitivity, and particularly in this group, the evolutionary pressures responsible are centred on unique polarisation signalling structures used in social interaction. Other functions of polarisation sensitivity in crustaceans include navigation, phototaxis and potentially increasing visual range through de-hazing in a turbid world