Effects of exposure to low, ecologically relevant doses of atrazine on somatic and gonadal development in American toad (Bufo americanus) tadpoles

摘要

Atrazine is the most commonly used herbicide in the United States, with 80 million pounds applied annually, making it the most common contaminant of ground and surface water nationwide. It has been shown to act as a potent endocrine disrupter in amphibians, causing altered somatic and gonadal development in the ecologically relevant part per billion range; as a result, it has been hypothesized that atrazine may be a major factor behind amphibian declines. However, responses of different species to the chemical vary widely, and have made predicting susceptibility difficult. Recently, it has been shown that life history can serve as a strong predictor of vulnerability, as the speed of somatic development and the timing of gonadal differentiation may determine the effects of exposure. However, previous studies leading to these conclusions have examined atrazine under stable laboratory conditions, although it is widely accepted that chemical contaminants can interact synergistically with natural stressors in the wild, producing exaggerated effects. To test whether more stressful conditions alter the effects of atrazine with respect to existing data, we raised Bufoamericanus tadpoles under more stressful conditions, including a high larval density and simulated pond drying, and exposure to ecologically relevant doses of atrazine (0, 0.1, 1, 25 ppb). We measured markers of somatic development (mass, time, survival at metamorphosis) and gonadal differentiation (ovarian stage in females, presence of testicular oocytes in males). Our results do not suggest that stressful conditions worsen the effects of atrazine, as only mass at metamorphosis was affected by exposure. Our results are interesting, however, in that they support the hypothesis that atrazine displays a non-monotonic dose-response curve, with very low concentrations (0.1,1 ppb) producing the most severe effects, an important implication for any conservation policy regarding the chemical.