SPATIAL VISION IN THE PROSOBRANCH GASTROPOD AMPULARIA SP

The eyes of gastropods of the genus Ampularia superficially resemble t he well-developed camera-type eyes of Littorina littorea and Strombus raninus. The eyes are of the closed-vesicle type, having a cornea and a lens that is separated from the retina by a narrow vitreous body. Li ght and electron microscopy were used to generate an accurate geometri cal model of the Ampularia sp, eye, which was then used to predict its optical performance. The image quality of the lens was investigated u sing a modified microscope and revealed that images suffer from severe aberrations. The focal length was estimated to be approximately 430 m u m, putting the plane of best focus in or just proximal to the rhabdo ms, The aberrant optics result in a large retinal blur-circle with a d iameter of approximately 120 mu m and an angular half-width of approxi mately 17 degrees, allowing only comparatively poor resolution compare d with the eyes of Littorina littorea and Strombus raninus. Behavioura l experiments revealed no significant optomotor response. The results imply that Ampularia sp. has poor spatial vision, limited by the blur- circles on the retina. The eyes appear to be suitable only for relativ ely simple visual tasks, such as finding an open water surface for bre athing, but the large size of the eye allows it to perform this task e ven at night.