The eye covering on snakes

The eye of all snakes is covered by a transparent spectacle that originates from the fusion of the eyelids during embryonic development. It is generally believed that the spectacle arose as an evolutionary adaptation to protect the eyes of early fossorial snakes. However, given that extant snakes occur in a variety of habitats with some species being almost exclusively fossorial, aquatic or terrestrial, and display diverse activity patterns where some species are nocturnal and others primarily active during the day, it is reasonable to expect substantial interspecies and adaptive differences in the requirement of the spectacle in terms of providing physical protection of the eye or partaking in visual optics.

Da Silva et al. (2017) have recently demonstrated an excellent correlation between measurements of spectacle thickness collected via optical coherence tomography (OCT), a non-contact medical imaging technology where reflected light is used to produce detailed cross-sectional images of biological tissue and those measured by light microscopy of formalin-fixed specimens. In order to determine variations in spectacle thickness among species, the spectacles of 217 alcohol-preserved museum specimens of 44 species belonging to 14 different families underwent optical coherence tomography (OCT) to measure spectacular thickness. Multivariable analyses were made to determine whether family, activity period (diurnal/nocturnal) and habitat (arboreal/terrestrial/fossorial/aquatic) influenced spectacle thickness.

The thinnest spectacle in absolute terms was found in the Usambara bush viper (Viperidae) with a thickness of 74 ± 9 μm and the absolute thickest spectacle was found in the red-tailed pipe snake (Cylindrophiidae) which had a spectacle thickness of 244 ± 57 μm. Fossorial and aquatic snakes had significantly thicker spectacles than arboreal and terrestrial snakes. When spectacle thickness was correlated to eye size (horizontal spectacle diameter), Gray’s earth snake (Uropeltidae) had the lowest ratio (1:7) and the cottonmouth (Viperidae) had the highest ratio (1:65). Multivariable and phylogenetic analyses showed that spectacular thickness could be predicted by taxonomic family and habitat, but not activity period.

This phylogenetically broad systematic study of the thickness of the snake spectacle showed that spectacular thickness varies greatly across snake species and may reflect evolutionary adaptation and development.

Citation
Da Silva MA, Heegaard S, Wang T, Gade JT, Damsgaard C, Bertelsen MF. 2017. Morphology of the snake spectacle reflects its evolutionary adaptation and development. BMC Veterinary Research. 13(1):258.