Photoreceptors and visual pigments in the red-eared turtle, Trachemys scripta elegans

Abstract

Absorbance spectra of cone outer segments and oil droplets were recorded microspectrophotometrically in the retina of the red-eared turtle, Trachemys scripta elegans. There are four cone visual pigments, with λmax = 617 nm (red sensitive), 515 nm (green sensitive), 458 nm (blue sensitive), and 372 nm (UV-sensitive). The red-sensitive pigment resides in single cones with red or orange oil droplets, and in both members of double cones. The principal member of the double cone contains an orange oil droplet, and the accessory member is droplet free. The green-sensitive pigment is situated in single cones with orange/dark yellow droplets. The blue-sensitive pigment is combined with the UV-absorbing oil droplet in single cones. The UV-sensitive pigment resides in single cones with clear oil droplets that exhibited virtually no absorbance down to 325 nm. Thus, seven types of cones can be identified based on their morphology, oil droplet color, and the visual pigment absorbance. At the moment, this is the most complex cone system described for vertebrates.