Blood flow dynamics in the snake spectacle

Abstract

SUMMARYThe eyes of snakes are shielded beneath a layer of transparent integument referred to as the 'reptilian spectacle'. Well adapted to vision by virtue of its optical transparency, it nevertheless retains one characteristic of the integument that would otherwise prove detrimental to vision: its vascularity. Given the potential consequence of spectacle blood vessels on visual clarity, one might expect adaptations to have evolved that mitigate their negative impact. Earlier research demonstrated an adaptation to their spatial layout in only one species to reduce the vessels' density in the region serving the foveal and binocular visual fields. Here, we present a study of spectacle blood flow dynamics and provide evidence of a mechanism to mitigate the spectacle blood vessels' deleterious effect on vision by regulation of blood flow through them. It was found that when snakes are at rest and undisturbed, spectacle vessels undergo cycles of dilation and constriction, such that the majority of the time the vessels are fully constricted, effectively removing them from the visual field. When snakes are presented with a visual threat, spectacle vessels constrict and remain constricted for longer periods than occur during the resting cycles, thus guaranteeing the best possible visual capabilities in times of need. Finally, during the snakes' renewal phase when they are generating a new stratum corneum, the resting cycle is abolished, spectacle vessels remain dilated and blood flow remains strong and continuous. The significance of these findings in terms of the visual capabilities and physiology of snakes is discussed. Supplementary material available online at

Scanning laser ophthalmoscopy and optical coherence tomography imaging of spectacular ecdysis in the corn snake (Pantherophis guttatus) and the California king snake (Lampropeltis getulus californiae)

“…The vascular lumens were not distinguishable by OCT at any time. In recent studies, van Doorn demonstrated that spectacular blood flow is low and variable in resting and undisturbed coachwhip snakes ( Masticophis flagellum , Colubridae), and becomes much increased and continuous during the integument renewal phase . This is concordant with our observations.…”

Spectral domain optical coherence tomography imaging of spectacular ecdysis in the royal python (Python regius)

“…Snakes possess a tertiary‐type spectacle that originates from the fusion of the upper and lower eyelid, thereby providing protection against injury and desiccation of the cornea, and allowing the snake to keep its eyes continuously open to detect predators and prey . Like other scales, the spectacle is vascularized and the cuticle of the spectacle is shed with the rest of the epithelium of the head during ecdysis. The spectacle also plays a critical optical role in refraction, almost equal to that of the lens .…”

Comparative morphology of the snake spectacle using light and transmission electron microscopy

“…The presence of blood vessels in the spectacle of snakes and the ability to constrict these vessels have previously been reported in detail . However, this study is the first to reflect on the importance of the location of the vessels within the spectacle stroma.…”

“…The position of vessels may influence the visual acuity of the snake. Species with slit pupils such as boas and pythons are speculated to be more susceptible to casting perceptible images of the spectacle vessels on the retina, whereas species with larger, round pupils such as many colubrids are less likely to perceive the spectacle vessels as entoptic phenomena . The location of the blood vessels of the spectacle in the middle or inner layers of the stroma may therefore be considered to be related to the area of minimal reflection of the vessels on the retina.…”

“…Previous studies have shown that the orientation of the blood vessels differs between species, so it would not be surprising if the location of the vessels also is species specific. A recent study stated that the vessels are maximally distended during the shedding phase and as the exact mechanism of shedding is unknown, there could be a possible connection between spectacular vessels and shedding. This present study did not examine the size of the blood vessels or whether they were blood filled.…”