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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 µg/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 thyroxine was elevated in both atrazine-exposed groups compared to the control group; plasma corticosterone was depressed in the 75µg/L treatment compared with both the control and 250 µg/L treatment. Larvae exposed to 75 µg/L atrazine reached stage 4 later, but at a size and weight comparable to the control group. By contrast, larvae in the 250 µg/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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Table of Contents

• Introduction
• Methods
• Results
• Discussion
• Acknowledgments
• Literature Cited

Tables and Figures

• Table 1. -- Summary of water quality characteristics of blended water used for exposure of tiger salamander larvae to atrazine.
• Table 2. -- Results of analysis of variance for differentiation and growth of tiger salamander larvae exposed to 0, 75, and 250 µg/L atrazine.
• Table 3. -- Results of ANOVA to partition variance in days-to-stage.

• Figure 1. -- Plasma corticosterone and plasma thyroxine concentrations in stage II and stage IV larvae exposed to 0, 75, and 250 µg/L atrazine.
• Figure 2. -- Days to stage, snout-vent length, and weight for larvae at stage II or stage IV exposed to 0, 75, and 250 µg/L atrazine.
• Figure 3. -- Model for regulation of differentiation by corticosterone and thyroxine in response to growth rate.

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Diane Larson, U.S. Geological Survey, Biological Resources Division, Northern Prairie Wildlife Research Center, Minnesota Field Station, 100 Ecology Building, 1987 Upper Buford Circle, Saint Paul, MN 55108
Susan McDonald and Steven J. Hamilton, U.S. Geological Survey, Biological Resources Division, Environmental and Contaminants Research Center, Field Research Station, RR 1 Box 295, Yankton, SD 57078-9214
Albert J. Fivizzani, University of North Dakota, Department of Biology, Grand Forks, ND 58202
Wesley E. Newton, U.S. Geological Survey, Biological Resources Divison, Northern Prairie Wildlife Research Center, Jamestown, ND 58401

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