Effects of the Herbicide Atrazine onAmbystoma tigrinumMetamorphosis: Duration, Larval Growth, and Hormonal Response

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Abstract
We exposed larval tiger salamanders (Ambystoma tigrinum) reared in the laboratory from eggs collected from a prairie wetland in North Dakota to three concentrations of atrazine (0, 75, and 250 mg/L) in a static renewal test to determine the pesticide's effect on (1) plasma corticosterone and thyroxine concentrations, (2) larval size, and (3) days‐to‐stage at stages 2 and 4 of metamorphic climax. We found significant effects of atrazine on each of these response variables. Plasma thyrox‐ine was elevated in both atrazine‐exposed groups compared with the control group; plasma corticosterone was depressed in the 75 mg/L treatment compared with both the control and 250 mg/L treatment. Larvae exposed to 75 mg/L atrazine reached stage 4 later but at a size and weight comparable to the control group. By contrast, larvae in the 250 mg/L treatment progressed to stage 4 at the same time but at a smaller size and lower weight than larvae in the control group. These results indicate that the herbicide has the potential to influence tiger salamander life history. We present a model consistent with our results, whereby corticosterone and thyroxine interact to regulate metamorphosis of tiger salamanders based on nutrient assimilation and adult fitness.

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