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/mm² in the centre vs. 2315–4291 cells/mm² 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.