With transparent, closed eyelids, what protects the snake eye from toxic solar ultraviolet radiation?

Title With transparent, closed eyelids, what protects the snake eye from toxic solar ultraviolet radiation?
Author, Co-Author Reyanna Inkster Henning, James Walsh, Jan Bergmanson, Stefanie Bergmanson
Program Number
Room 201-203
Abstract Purpose: Snakes habituate geographic regions with high intensity solar irradiance, but as their eyes develop with embryonic clear eyelids that remain fused throughout life, they may lack the light protection expected by eyelids in other species. This study assessed whether the lenticule, the clear snakeskin scale over the eye, performs this ultraviolet radiation (UVR) protective role.

Methods: Naturally shed skins from 11 species of venomous snakes from a broad range of global habitats were examined. Spectral transmittance of snake lenticules and non-lenticule scales were measured using a spectrometer system with suitably adapted front end optics.

Results: Inter-species spectral transmittance curves were quantified and compared. All lenticules absorbed the most toxic UVC waveband (100-280 nm) which in solar irradiance is filtered by atmospheric ozone, typically solar irradiance drops to near zero at around 290 nm. For many species UVB (280-315 nm) was absorbed below 300 nm with transmittance reduced to 50% near 325 nm but some species had a higher cut-off wavelength transmitting almost no UVB.  The UVA band was not absorbed in any of the species examined. The non-lenticule scales frequently followed a similar pattern except for the more pigmented snakes where the pigment flattened the curves in the visible waveband.

Conclusions: The lenticule is an incomplete UVR blocker that partially filtered out the more toxic UVB and UVC wavelengths but would not meet the ANSI Class 2 standard. Older snakes are known to develop cataract and this may be explained by the insufficient UVR protection provided by the transparent eyelids and here demonstrated UVR transmittance of the lenticule. The amount of toxic radiation transmitted and allowed to reach the retina is controlled by the crystalline lens and possibly the eyelids, neither structure was assessed in the present study. The non-lenticule scales of the skin had transmittance curves similar to the lenticule suggesting that the lenticule did not evolve to offer additional UVR protection.
Affiliation of Co-Authors MAHSA University, Texas Eye Research and Technology Center, UHCO, UHCO