Visual filtering in box jellyfish: A simple system

Unlike most jellyfish, box jellyfish (cubomedusae) are highly visually orientated. They are agile swimmers and use visual information to avoid obstacles and orientate themselves. They have an elaborate visual system which consists of four sensory structures called rhopalia, each holding six eyes of four morphological types: 2 camera-type eyes (upper and lower lens-eyes) and 2 lesser pigment cup eyes (pit and slit eyes). The variety of eye types combined with the relative sparsity of their nervous system makes it likely that the visual system of box jellyfish is a collection of special purpose eyes i.e. eyes which perform one or few visual tasks. The specialisation of special purpose eyes, allow them to heavily filter visual information very early in a visual system and consequently, they require fewer second order visual neurons Work in this thesis has been carried out with the intent to gain further understanding of the role of vision in box jellyfish. A variety of methods have been employed to investigate various aspects of vision, including histology, optical modeling, electrophysiology, microspectrophotometry and behaviour. Using these methods, spatial, temporal and spectral filtering of visual information as well as visual behaviour and adaptations to different light intensities have been examined. Experiments described in Chapter I and II show that there are differences in the morphology as well as filtering mechanisms in the spatial and temporal domains between the upper and lower lens-eye. Additionally, we find that box jellyfish have low temporal and spatial resolution. These results support the hypothesis that the respective lens eyes are special purpose eyes and correspondingly mediate different visual tasks. The work carried out in Chapter IV reveals a visual task mediated by the lower lens-eye in both T. cystophora and C. bronzie, obstacle avoidance. The ability to avoid obstacles differs between the species tested, reflecting the differences in spatial resolution between the species revealed in Chapter I. In Chapter III, microspectrophotometry combined with immunohistochemistry show that the lens eyes in the species C.bronzie have one visual pigment with maximal sensitivity of approximately 510 nm. Microspectrophotometry also indicates that the visual pigment of box jellyfish undergo bleaching, a characteristic common to vertebrate pigments but not invertebrates