Spectral sensitivities of seven morphological types of photoreceptors in the retina of the turtle, Geoclemys reevesii

Abstract

Spectral sensitivities of photoreceptors in the turtle (Geoclemys) retina were studied by intracellular recording, and each cell was filled with Lucifer yellow (LY). Photoreceptors were classified into seven morphological types: rod, four types of single cones, and two members of a double cone. Single cones contained one of four different oil droplets: red, pale-green, orange, and clear. Double cones consisted of two apposed cones; principal members contained yellow oil droplets, while accessory members contained no oil droplet. Spectral sensitivities recorded from these seven types of photoreceptors were classified into one type of rod and three chromatic types of cones. Rods (n = 19) showed peak sensitivity at 520 nm. Single cones containing either a red (n = 51) or a pale-green (n = 9) oil droplet were red-sensitive (λmax at 620 nm). Single cones containing an orange oil droplet (n = 14) were green-sensitive (λmax at 540 nm). Single cones containing a clear oil droplet (n = 3) were blue-sensitive (λmax at 460 nm). Both members of the double cone, principal (n = 22) and accessory (n = 15), were red-sensitive (λmax at 620 nm) No diffusion of LY was detected between the apposed members of double cones. Red-sensitive cones, therefore, consisted of four different morphological types of cones, and they occupy about 70% of the photoreceptor mosaic in the turtle retina.