Retinal characteristics of the ornate dragon lizard, Ctenophorus ornatus

Abstract

The retina of a diurnal insectivorous lizard, Ctenophorus ornatus (Agamidae) was investigated using microspectrophotometry and light and electron microscopy. A prominent broad yellow band was observed that extended across the mid-retina. The yellow coloration was found to originate from both oil droplets and diffuse pigmentation within cone inner segments. Microspectrophotometric analysis revealed yellow oil droplets with variable absorption of wavelengths below 520 nm and transparent oil droplets with no detectable absorptance between 350 and 750 nm. Cones with transparent oil droplets lacked the diffuse yellow pigmentation. The mean wavelengths of maximum absorbance of visual pigments in the isolated cone outer segments were at 440, 493, and 571 nm. The retina was found to possess a deep convexiclivate fovea located within the yellow band, slightly dorsotemporal of the retinal midpoint. The topography of the retinal ganglion cells revealed that the fovea was contained within an area centralis. Photoreceptors were either single (80%) or unequal double (20%) cones. Within the region of the fovea, the cones were approximately 20% the diameter of those in the peripheral retina. Colored oil droplets and yellow pigment may increase visual acuity by absorbing short wavelength light scattered either by the atmosphere or the optical structures of the eye. The presence of a fovea containing slender cone photoreceptors and three visual pigments suggests that the lizard has high acuity and the potential for color vision.