The anatomy and physiology of selected reflecting superposition eyes.
2015-11-19T08:52:32Z (GMT) by
Aspects of the morphology and optical physiology of reflecting superposition eyes have been investigated using species from various decapod crustacean taxa. The eyes all have the same basic structure of a distal dioptric layer and a proximal retinula layer, separated by an unpigmented clear zone. In the eyes of shrimps, lobsters and crayfish the clear zone is crossed by crystalline cone cell extensions. In squat lobsters this region is crossed by rhabdomeric lightguides. Porcelain crabs possess an eye intermediate in design. The superposition ray path, the action of corneal lenses and the presence of lightguides have been demonstrated and the refractive indices of several optical elements determined. These results have been used to produce ray tracing diagrams showing the optical pathways within these eyes. All of the eyes function by redirecting light across the clear zone by reflection within a mirrored crystalline cone. In squat lobsters a rhabdomeric lightguide transmits axial light to the rhabdoms. The eyes have a small f-number resulting in good light-gathering power and maximum sensitivity. However, intracellular electrophysiological determinations of retinula cell angular sensitivity show that these eyes have poor resolution. Variations in morphology and optics represent adaptations to the underwater light field, especially in the tapeta and in the light- sensitive rhabdoms. In oceanic species the tapetum varies in both structure and reflectivity along a dorso-ventral gradient within each eye and also interspecifically. It is proposed that the need to remain well-camouflaged in the low-contrast oceanic environment leads to tapetal modifications. Where sufficient light is available the rhabdoms are adapted to maximize resolution and permit sensitivity to polarized light. In deeper water, where little light remains, the rhabdoms are adapted to increase sensitivity at the expense of resolution. The dorsal region of the eye retains apposition optics for the purpose of detecting small objects in silhouette against the downwelling light.