Variation in somatic and ovarian development: predicting susceptibility of amphibians to estrogenic contaminants.

Although amphibian sex determination is genetic, it can be manipulated by exogenous hormone exposure during sexual differentiation. The timing of sexual differentiation varies among anuran amphibians such that species may or may not be a tadpole during this period, and therefore, may or may not be exposed to aquatic contaminants. Estrogenic contamination is present in amphibian habitats worldwide. We examined three species with varying somatic and ovarian developmental rates to assess their susceptibility to estrogenic contaminants. American toads (Bufo americanus), gray treefrogs (Hyla versicolor), and Southern leopard frogs (Ranasphenocephala) were exposed as larvae to 17-beta-estradiol (10(-7)M), three concentrations of a widespread herbicide (1, 3, 30 ppb atrazine), or a solvent control (ethanol). Somatic and ovarian developmental stages as well as time to metamorphosis were recorded. Toads and treefrogs were examined at three weeks and metamorphosis, while leopard frogs were examined at three, six, and nine weeks as well as at metamorphosis. Our results demonstrate that each species displays heterochronic somatic and ovarian development. Further, the more rapid of the two rates determines the susceptibility to estrogenic contaminants. These results suggests that amphibians with shorter larval periods, and therefore quicker somatic developmental rates (i.e. American toads, gray treefrogs), are more susceptible to somatic treatment effects (i.e. prolonged time to metamorphosis) due to estrogenic contaminants. Moreover, the results suggest that amphibians with relatively rapid ovarian development (i.e. Southern leopard frogs) are more susceptible to gonadal treatment effects caused by estrogenic contaminants.

Survivorship patterns of larval amphibians exposed to low concentrations of atrazine.

Amphibians can be exposed to contaminants in nature by many routes, but perhaps the most likely route is agricultural runoff in amphibian breeding sites. This runoff results in high-level pulses of pesticides. For example, atrazine, the most widely used pesticide in the United States, can be present at several parts per million in agricultural runoff. However, pesticide levels are likely to remain in the environment at low levels for longer periods. Nevertheless, most studies designed to examine the impacts of contaminants are limited to short-term (approximately 4 days) tests conducted at relatively high concentrations. To investigate longer-term (approximately 30 days) exposure of amphibians to low pesticide levels, we exposed tadpoles of four species of frogs--spring peepers (Pseudacris crucifer), American toads (Bufo americanus, green frogs (Rana clamitans), and wood frogs (Rana sylvatica)--at early and late developmental stages to low concentrations of a commercial preparation of atrazine (3, 30, or 100 ppb; the U.S. Environmental Protection Agency drinking water standard is 3 ppb). We found counterintuitive patterns in rate of survivorship. Survival was significantly lower for all animals exposed to 3 ppb compared with either 30 or 100 ppb, except the late stages of B. americanus and R. sylvatica. These survival patterns highlight the importance of investigating the impacts of contaminants with realistic exposures and at various developmental stages. This may be particularly important for compounds that produce greater mortality at lower doses than higher doses, a pattern characteristic of many endocrine disruptors.