74.4  Wednesday, Jan. 6  The effects of excessive dietary mercury on thyroid-mediated processes and fitness- related traits in wood frog tadpoles WADA, H*; BERGERON, CM; MCNABB, FMA; TODD, BD; HOPKINS, WA; Virginia Tech hwada@uwo.ca

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, a process mediated by thyroid hormones (TH), in southern leopard frogs. However, the 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 ug/g, dry wt) on TH concentrations and development, growth, performance, and survival of wood frogs (Rana sylvatica). We predicted that TH concentrations in individuals exposed to dietary Hg would be reduced, resulting in developmental delays, particularly near metamorphic climax when TH levels typically peak. Tadpoles accumulated Hg in a dose-dependent manner; total Hg concentrations in Gosner stage 42 tadpoles fed the three diets were 0.03, 1.06, 3.54 ug/g, dw, respectively. During metamorphic climax, tadpoles eliminated 35% of the inorganic Hg from their tissues but retained most of the more toxic methylmercury that they had accumulated. Contrary to our predictions, we found no effect of Hg on the duration of larval 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 TH concentrations among our dietary treatments. Our results suggest that amphibian species may differ in their sensitivity to dietary Hg, perhaps due to differences in the length of the larval period or other life history factors.