Evidence of Retinal Light Damage in Rana cascadae: A Declining Amphibian Species

Abstract

Diurnal wildlife populations that inhabit high-altitude environments may be at risk of cumulative retinal injury from increased penetration of solar ultraviolet radiation resulting from a declining concentration of stratospheric ozone. Previously, the deleterious effect of ultraviolet radiation on the eggs and hatching success has been shown in Rana cascadae, a high-altitude species undergoing major population declines. To assess whether the retinas of this species may be showing signs of injury resulting from cumulative exposure to solar radiation, both the histological condition and photoreceptor densities were compared in R. cascadae, in experimentally light-damaged R. pipiens, and in control R. pipiens. Distinctive outer-retinal abnormalities observed in the inferior retina of R. cascadae were similar to those observed in light-damaged R. pipiens. These included an abnormal distribution of retinal pigment epithelial melanin, damaged photoreceptors and the presence of large, pigment-filled macrophages. Statistically significant, negative correlations were found between extent of outer-retinal histopathology and cone photoreceptor densities both in light-damaged R. pipiens (r = -0.54) and R. cascadae (r = -0.59). Overall cone photoreceptor densities were lower in the inferior versus superior retina of R.cascadae and in light-damaged R. pipiens compared with controls. In R. pipiens, experimental light damage significantly reduced green-rod and miniature-cone densities in the inferior retina; both contain a short-wavelength sensitive photopigment. The lowest densities of these two photoreceptor types were found in the inferior retina of R. cascadae as well. Similarities between the major abnormalities observed in R. cascadae and in experimentally light-damaged R. pipiens suggest that environmental solar radiation is damaging the retinas of R. cascadae and perhaps other highaltitude species as well.