Understanding the cephalic eyes of pulmonate gastropods: A review

This review showcases one group of gastropod's ability to preceive light through the eyes. The central question is simple: what are the visual performances and tasks of cephalic eyes in gastropods? That topic in itself is rather broad and is here applied to pulmonate gastropods, coming from terrestrial and aquatic biomes as well as different habitats and microhabitats, exhibiting different life-styles and light-tolerances. Therefore, the main objectives have been to analyze (1) anatomical and ultrastructural eye characteristics, (2) optical systems, (3) image-forming capabilities and possible functional consequences of eye size and design, (4) interactions between gastropods and their environment mediated by the visual information obstained through the eye, and (5) the specific visual tasks that the eyes serve. During the course of this study, a range of variations (= adaptations) in both optical and retinal clesipi parameters, including eye size, aperture size, quality of optical image, retinal shape, sampling density, and optical sensitivity were discovered. All species of pulmonate gastropods studied have paired simple camera-type eyes that operate with advanced fixed focal-length optics. However, in terrestrial snails and slugs as well as freshwater limpets, the optics cannot produce a focused image on their shallow retinas. This seems to indicate that eyes in these species are not designed to receive a focused image and are likely to measure only the average light intensity or quality over large angles rather than resolve fine image details. The aquatic snails examined are able to focus a sharp image on the photoreceptive layer of the retina dote to the deepenings of the latter (at least in a localized region). Although there is a significant correlation between specialization of the eye (e.g., quality of optical image, sensitivity, and resolution) for a particular visual task in a specific habitat that the species encounters, there is no correlation between cellular composition of the retina and light/dark preferences. Their high optical sensitivity allows terrestrial snails to perform the necessary visual tasks in both bright and dim light, whereas the eye in aquatic species functions preferentially under bright light conditions. In conclusion, pulmonate gastropods use their eyes primarily for the following two kinds of visual tasks: (1) discriminating objects and possible enemies in their environment and (2) monitoring the environmental brightness level to orient towards dark places. The first type of visual task is characteristic of the aquatic snails and is served by image-forming eyes; the second is typical of terrestrial snails and slugs and is best served by a blurred image. Attention is given to visual ecological adaptations, specific visual needs, and the evolutionary history of gastropods.