Circular polarization vision in stomatopod crustaceans

Perhaps due to relative speed and reliability, the information that light provides is useful to many animals. As a result, visual systems are a major, sensory system in several animal groups. Various aspects of vision become tuned by the environment and behavioural need, including relative sensitivity, speed, colour, and in some species, polarization. The stomatopod crustaceans (mantis shrimps) are remarkable in that they possess, at the retinal level, the most complex colour system known including 12 spectral sensitivities. However, like other crustaceans their lifestyles may also rely on the information they can receive from polarization. This is evident from the range of polarization sensitive areas of their subdivided eye. The hemispheric regions of the stomatopod eye are linear polarization sensitive, in a total of four different e-vector orientations. Additionally, stomatopods are able to rotate their eyes making the actual angle of polarization sensitivity to the outside world arbitrary. Perhaps most interesting in the context of polarization vision is the function of the eighth retinular cells, known as R8, of rows 5 and 6 of the midband region. These birefringent cells act as quarter-wave retarders allowing stomatopods to be sensitive to circularly polarized light (CPL). This ability has yet to be demonstrated in other animals and compared to linear polarization vision (LPV), circular polarization vision (CPV) is not well understood. The aim of this thesis was to further investigate rows 5 and 6 of the midband in a range of stomatopod species, using both behavioural and anatomical techniques. Firstly, through modelling the birefringent properties of the R8 in six species, it is revealed that the ability to discriminate CPL is shared among many stomatopods (Chapter 2). Adaptive pressure to maintain the size of the R8 cell, achieving quarter-wave retardance, suggests that it provides an important role for stomatopods. However, one species included in this study, Haptosquilla trispinosa, is likely sensitive to a different ellipticity of polarized light than circular. Since its discovery in 2008, it was hypothesized that circular polarization vision could provide a covert communication system for stomatopods. Chapter 3 uses behavioural paradigms to examine the discrimination abilities of Gonodactylaceus falcatus, a species of stomatopod with extensive polarization patterns on their bodies, and demonstrates that they use circular polarization as a signal for burrow occupancy. However, as variation in body patterns exist between species, with some species having none at all, other roles for CPL need to be considered. H. trispinosa is one species of stomatopod that lacks circularly polarized reflections, and the birefringent properties of their R8 cells, presented in Chapter 2, favours sensitivity to a form of elliptically polarized light, not circular. Behavioural discrimination tests iii confirm the presence of elliptical polarization vision (EPV) in this burrow dwelling species and further behavioural testing provided possible uses for this new polarization sensitivity. Understanding how an animal uses an ability like CPV, or EPV, when humans are insensitive to this property of light present many challenges. While the retinal anatomy of stomatopods is relatively well understood the neural processing of visual information, a vital part of this understanding, has been relatively neglected in stomatopods. The final research chapter of this thesis (Chapter 5) uses serial blockface scanning electron microscopy to investigate the first optic neuropil, the lamina. A three-dimensional reconstruction of individual lamina cartridges allows identification of cell types and pathways below the retina and provides a new method for use in stomatopod neural architecture investigations. The research presented in this thesis provides evidence contrary to the assumption that row 5 and 6 of the midband provide all species with CPV, despite appearing anatomically identical. We present a number of alternative hypotheses for the function of these rows, highlighting the likelihood that for different species specific roles exist.