Developmental Responses of Amphibians to Solar and Artificial UVB Sources: A Comparative Study

Abstract

Many amphibian species, in widely scattered locations, currently show population declines and/or reductions in range, but other amphibian species show no such declines. There is no known single cause for these declines. Differential sensitivity to UVB radiation among species might be one contributing factor. We have focused on amphibian eggs, potentially the most UVB-sensitive stage, and compared their resistance to UVB components of sunlight with their levels of photolyase, typically the most important enzyme for repair of the major UV photoproducts in DNA, cyclobutane pyrimidine dimers. Photolyase varied 100-fold among eggs/oocytes of 10 species. Among three species-Hyla regilla, Rana cascadae, and Bufo boreas-for which resistance of eggs to solar UVB irradiance in their natural locations was measured, hatching success correlated strongly with photolyase. Two additional species, Rana aurora and Ambystoma gracile, now show similar correlations. Among the low-egg-photolyase species, R. cascadae and B. boreas are showing declines, and the status of A. gracile is not known. Of the two high-photolyase species, populations of H. regilla remain robust, but populations of R. aurora are showing declines. To determine whether levels of photolyase or other repair activities are affected by solar exposures during amphibian development, we have initiated an extended study of H. regilla and R. cascadae, and of Xenopus laevis, laboratory-reared specimens of which previously showed very low photolyase levels. Hyla regilla and R. cascadae tadpoles are being reared to maturity in laboratories supplemented with modest levels of UV light or light filtered to remove UVB wavelengths. Young X. laevis females are being reared indoors and outdoors. Initial observations reveal severe effects of both UVA and UVB light on H. regilla and R. cascadae tadpoles and metamorphs, including developmental abnormalities and high mortalities. Assays of photolyase levels in the skins of young animals roughly parallel previous egg/oocyte photolyase measurements for all three species.

Effects on aquatic ecosystems

“…Amphibian populations are in serious decline in many areas of the world [151], and scientists are seeking explanations for this phenomenon [152,153]. Worrest and Kimeldorf [154] noted several adverse effects of increased exposure to UV-B radiation on the systemic development of boreal toad (Bufo boreas boreas) tadpoles in the laboratory.…”

A small increase in UV-B increases the susceptibility of tadpoles to predation

“…One study that has done so demonstrated that UV-B-induced spinal curvature in tadpoles can ultimately lead to death by causing them to swim in circles and thus preventing them from foraging [11]. Similarly, other UV-B-induced malformations and abnormalities such as oedema or bloating [21,22], eye abnormalities [23,24] and hindlimb malformations [12] may also negatively affect foraging ability, predator evasion and reproductive success, but, critically, these possibilities have not been tested directly. Other studies have measured correlates of fitness such as locomotor performance, predator-induced morphological defences, anti-predator behaviour, and age and size at metamorphosis [25 -31], and have demonstrated that these are negatively impacted by exposure to UV-B [10,14,16,32], but, again, none of these studies measured the fitness variables that these parameters are meant to be correlated to (i.e.…”

Octylphenol and UV-B radiation alter larval development and hypothalamic gene expression in the leopard frog (Rana pipiens).

“…Additionally, the 1-nM OP exposure is approximately 2,800 times less than the calculated LC 50 for R. pipiens tadpoles. The observed effects of UV-B on gene expression are in addition to the well-characterized effects of this stressor on several aspects of amphibian development and survival (11)(12)(13)(14).…”

“…Severe deformities including lordosis (curvature of the spine), bloating/distension, and abnormal development of the presumptive cornea were common pathologies associated with UV-B exposure (13,14). Ankley et al (15) reported symmetrical hind-limb ectromelia and ectrodactyly after R. pipiens tadpoles were exposed to low levels (44 µW/cm 2 ) of UV-B.…”

“…It has been hypothesized that exposure of amphibians to pollutants may disrupt endocrine systems controlling metamorphosis, stress response, and sexual development (5)(6)(7)(8)(9)(10). Exposure to UV-B radiation (280-320 nm) has also been found to disrupt development in several amphibian species (11)(12)(13)(14)(15). The vulnerability of amphibian species to environmental stressors such as EDCs and UV-B may be associated with amphibian population declines that have been occurring worldwide since the 1960s (16).…”