Dietary Mercury Has No Observable Effects on Thyroid-Mediated Processes and Fitness-Related Traits in Wood Frogs

Abstract

Mercury (Hg) is a neurotoxicant known to cause developmental and behavioral abnormalities in vertebrates. Increasing evidence suggests that Hg can also disrupt endocrine functions and endocrine-dependent processes. For example, dietary Hg has been shown to delay tail resorption during metamorphic climax in amphibians, a process mediated by thyroid hormones. However, a direct link between Hg, hormone disruption, and developmental delays in amphibians has not been explored. Therefore, we examined the effects of dietary Hg (0.01, 2.5, and 10 μg/g total Hg, dry wt) on thyroid hormone concentrations, development, growth, performance, and survival of wood frogs (Rana sylvatica). Tadpoles accumulated Hg in a concentration-dependent manner; total Hg concentrations in tadpoles at the beginning of metamorphic climax (Gosner stage 42) were 0.03, 1.06, 3.54 μg/g, dry wt, for control, low, and high Hg diets, respectively. During metamorphic climax, tadpoles eliminated 35% of the inorganic Hg from their tissues but retained most of their accumulated methylmercury. Contrary to our predictions, we found no effect of Hg on the duration of tadpole development, size at metamorphosis, tail resorption time, or hopping performance. Consistent with the lack of effects on development, we also detected no differences in whole-body thyroid hormone concentrations among our dietary treatments. Our results, when compared with the effects of Hg on other amphibians, suggest that amphibian species may differ substantially in their sensitivity to dietary Hg, emphasizing the need for data on multiple species when establishing toxicity benchmarks.

Complex Life Histories Alter Patterns of Mercury Exposure and Accumulation in a Pond-Breeding Amphibian

“…We found diet and life cycle events are important drivers of MeHg and %MeHg across ontogeny of the boreal chorus frog. Many metal concentrations are greater premetamorphosis, but our results match previous work showing maternal transfer of Hg and retention and increase through metamorphosis , and adulthood similar to those observed with selenium ,, and strontium . The MeHg concentrations were consistent within life stages between the two ponds, despite the base of the food webs appearing to be quite different.…”

“…Amphibians face an array of conservation threats, including exposure to contaminants such as Hg. ,− Research suggests that tadpoles can eliminate inorganic Hg from their tissues but retain MeHg at metamorphosis . If bioaccumulated MeHg concentrations are high enough, development and metamorphic success can be adversely affected .…”

“…Metamorphosis is accompanied by large changes in morphology and physiology, requires an extraordinary energetic expense, and can exacerbate the toxicity of contaminants . Amphibians retain Hg during metamorphosis, , and sensitivity to MeHg exposure is heightened during this major developmental change . Thus, frogs are contending with the stress of metamorphosis while their internal concentration of a potent neurotoxin simultaneously doubles, potentially exacerbating existing health challenges associated with multiple stressors.…”

“…We assessed Hg concentrations (total Hg [THg], MeHg), isotopic compositions (δ 13 C, δ 15 N), and energy density in breeding adults (following hibernation), eggs, tadpoles, metamorphs (in the final stages of metamorphosis), and midsummer adults. While a few studies have investigated the changes in concentrations of trace metals and Hg across some portions of amphibian life history, − the mechanisms driving these changes are poorly characterized.…”