Surface Topography and Ultrastructure of the Spectacular Cells in the Eyes of Land and Sea Snakes (Squamata, Reptilia): Functional Adaptations of Micro-Ornamentation.

Although the surface micro-ornamentation of the scales within the skin of snakes has been the subject of many previous studies, there has been little work done on the spectacle, a protective (keratinised) goggle separated from the underlying cornea by a sub-spectacular space. The surface ultrastructure of the "Oberhäutchen" of the spectacle is examined in nine species of snakes (five aquatic and four terrestrial) using light and electron microscopy, micro-computed tomography and gel-based profilometry. Significant topographic differences in cell size (increases of between 5.4% and 165% in the periphery), shape (central pentagonal/hexagonal to long peripheral) and density (2579-10,391 cells/mm in the centre vs. 2315-4291 cells/mm in the periphery) are revealed. Small indentations in the surface (micropits) and/or microholes in the cell membrane decorate the epithelial surface of all species, which also show a centre-to-periphery gradient in diameter (42.39-120.55 nm in the centre vs. 63.76-182.60 nm in the periphery). Microridges are found on the superficial cells of the spectacle of only one species (the terrestrial Cantil Viper, Agkistrodon bilineatus) with straight, parallel ridges in the centre (138.4 ± 28.2 nm wide) and a more complex pattern of ridges (143.1 ± 19.1 nm wide) in the periphery. The micro-ornamentation of the spectacle in both land and sea snakes is compared with those found over the body scales and discussed with respect to a range of potential functions, while still allowing a clear optical pathway for vision.