THE PHOTORECEPTORS AND VISUAL PIGMENTS OF THE GARTER-SNAKE (THAMNOPHIS-SIRTALIS) - A MICROSPECTROPHOTOMETRIC, SCANNING ELECTRON-MICROSCOPICAND IMMUNOCYTOCHEMICAL STUDY

Scanning electron microscopy, immunocytochemistry, and single cell mic rospectrophotometry were employed to characterize the photoreceptors a nd visual pigments in the retina of the garter snake, Thamnophis sirta lis. The photoreceptor population was found to be comprised entirely o f cones, of which four distinct types were identified. About 45.5% of the photoreceptors are double cones consisting of a large principal me mber joined near the outer segment with a much smaller accessory membe r. About 40% of the photoreceptors are large single cones, and about 1 4.5% are small single cones forming two subtypes. The outer segments o f the large single cones and both the principal and accessory members of the doubles contain the same visual pigment, one with peak absorban ce near 554 nm. The small single cones contain either a visual pigment with peak absorbance near 482 nm or one with peak absorbance near 360 nm. Two classes of small single cones could be distinguished also by immunocytochemistry and scanning electron microscopy. The small single cones with the 360-nm pigment provide the garter snake with selective sensitivity to light in the near ultraviolet region of the spectrum. This ultraviolet sensitivity might be important in localization of phe romone trails.