Relationship between circadian habits and visual functions in diurnal and nocturnal snakes (snakes, Colubridae)

Abstract

The group of snakes presents numerous ecological adaptations and great morphological diversity. Studies of the types of photoreceptors present in the retina of snakes, their topographic density and functional physiology are important for the understanding of behavioral patterns, ecological and adaptive aspects of the species, which differ according to their visual needs and their habitat. These studies are also important to aid in understanding the evolutionary history of this group. The snake retina has different types of photoreceptors: rods and different groups of cones. The colubrid snakes have three types of cones, a large simple cone, a small simple cone and a double cone. The vertebrates suffered variations in the sensitivity of their visual pigments during evolution, which determined the type, and the spectral sensitivity of the visual pigments present in the retina of this group. Five classes of retinal visual pigments, RH1, RH2, SWS1, SWS2 and MWS/LWS were identified based on the amino acid sequence, molecular phylogeny and spectral sensitivity. The RH1 gene is generally expressed in rods that have rhodopsin as pigment; the other classes of opsins are expressed in the cones. Colubrid snakes present a special adaptation in their retinal morphology. Some studies have shown the absence of a typical rod in colubrid snakes with diurnal habits and the presence of four morphological types of cones. A group of double cones and a group of large simple cones, both containing the LWS photopigment, sensitive to long wavelengths and two groups of small cones, one sensitive to short wavelengths with the SWS1 photopigment, and a second group, sensitive to medium wavelengths, containing the typical RH1 photopigment of rods. Recent studies using immunohistochemical techniques and transmission electron microscopy indicate that the last type of photoreceptor is a modified rod, which has a cone-like morphology, and contains the typical RH1 photopigment. However, no study in the literature has investigated the functional characteristics of this group of photoreceptors. We will use the electroretinogram (ERG) to characterize and compare the photopic and scotopic responses of snakes with diurnal and nocturnal habits of the colubridae family. Through the characterization and description of the ERG, we can compare the electrophysiological results with results of morphological studies of retinas, and verify the possible differences between the responses obtained for snake with diurnal and nocturnal habits. Considering the absence of a typical rod in the retina of diurnal snakes, the investigation of functional characteristics and photopic and scotopic responses of this group of vertebrates have a great scientific interest. The electrophysiological studies to be carried out in this project will bring new and valuable information on the functional aspects in the retinas of diurnal and nocturnal snakes.