Effects of the herbicide atrazine on Ambystoma tigrinum metamorphosis: Duration, larval growth, and hormonal response

We exposed larval tiger salamanders (Ambystoma tigrinum) reared in the labo ratory from eggs collected from a prairie wetland in North Dakota to three concentrations of atrazine (0, 75, and 250 mu g/L) in a static renewal test to determine the pesticide's effect on (1) plasma corticosterone and thyro xine 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 with the control group; plasma corticosterone was depressed in the 75 mu g/L treatment compared with both the control and 250 mu g/L treatment. Larvae exposed to 75 mu g/L atrazine reached stage 4 lat er but at a size and weight comparable to the control group. By contrast, l arvae in the 250 mu g/L treatment progressed to stage 4 at the same time bu t at a smaller size and lower weight than larvae in the control group. Thes e results indicate that the herbicide has the potential to influence tiger salamander life history. We present a model consistent with our results, wh ereby corticosterone and thyroxine interact to regulate metamorphosis of ti ger salamanders based on nutrient assimilation and adult fitness.