Aspects of the visual function and adaptation of deep-sea decapods

This thesis attempts to relate the structure and physiology of superposition compound eyes of mesopelagic and bathypelagic decapods to their visual environment. An anatomical survey at the light microscope level showed that bathypelagic decapods have stellate or hypertrophied rhabdoms and little or no proximal shielding pigments. In contrast, species found in upper mesopelagic depths have fusiform rhabdoms that are square in cross section in the dorsal region of the eye and often have stellate rhabdoms ventrally. Proximal shielding pigment was much more noticeable in mesopelagic species than in bathypelagic species. In addition, shielding pigments were only found distal to the basement membrane in mesopelagic species. Observation of eyeshine intensity distribution around the eye confirmed histological evidence that the reflective tapetum is restricted to ventral regions of the eye in mesopelagic species while bathypelagic species have a complete tapetum. Measurements of aperture diameters using green light and white light showed that they are significantly smaller under green light. Since most mesopelagic species will rarely encounter anything other than blue/green light, the functional aperture of most superposition eyes is probably less than has been estimated previously. Physiological evidence suggests that spectral sensitivity and the ability of the eye to respond to rapid changes in light intensity changes with light adaptation in coastal species. Deep-sea species appear unable to light adapt. It is suggested that one reason for this may be the limited ability of the rhabdomeral membrane to respond to the increased rate of ion cycling induced by elevated light levels. This, and previous evidence, may also explain the ease with which the eyes of deep-sea decapods become damaged upon exposure to relatively low light intensities. A ray-tracing model was formulated that predicted the sensitivity and acceptance angle for all the species examined histologically. This agreed well with previous empirical models and suggested that the dioptric layer focuses light from a point source onto the axial rhabdom. The model suggests that sensitivity to extended sources increases with depth. It is suggested that this is an associated (concomitant) rather than causal relationship and that the eyes of mesopelagic and deep-sea decapods are primarily designed to maximise sensitivity to `point' sources such as bioluminescent events.