Comparative study of the visual system of two psammophilic lizards (Scincus scincus &Eumeces schneideri)

Sand deserts are common biotopes on the earth's surface. Some specialized vertebrate species have colonized these ecological habitats by living buried in the sand. Among these so called psammophilic species are the Scincidae sand dune living species Scincus scincus and Eumeces schneideri. These two skinks share a relatively similar behavioral ecology by living buried in sand, almost all the time for S. scincus and at least for some part of the day for E. schneideri. The visual system of these two lizards was investigated by histological, immunohistochemical, Magnetic Resonance Imaging (MRI) and morphometric techniques. Both skink species exhibit a retina lacking fovea, composed predominantly of cones presenting two types of oil droplets (pale blue-green and colorless). Both species possess a subset of rod like-photoreceptors (about 1 rod for 30 cones) evidenced by anti-rhodopsin immunoreactivity. A ratio 1:1-1:2 between ganglion cells and photoreceptors points to a linear connection (photoreceptors/bipolar neurons/ganglion cells) in the retina and indicates that both skinks more likely possess good visual acuity, even in the peripheral retina. The MRI analysis revealed differences between the species concerning the eye structures, with a more spherical eye shape for S. scincus, as well as a more flattened lens. The relative lens diameter of both species seems to correspond to a rather photopic pattern. Beside the fact that S. scincus and E. schneideri have different lifestyles, their visual capacities seem similar, and, generally speaking, these two psammophilic species theoretically exhibit visual capacities not far away from non-fossorial species.