Tadpoles of the African clawed frog (Xenopus laevis) were exposed to sublethal concentrations of atrazine (0, 40, and 320 microg/L) and nitrate (0, 37, and 292 mg/L) from feeding stage to metamorphosis. A 3 x 3 factorial design was used to identify both single and interactive effects. At metamorphosis, tadpole weight, snout-vent length (SVL), and hematocrit were determined. Mean mortality was greater in tanks receiving 320 microg/L atrazine; nitrate had no effect on mortality. Significant differences for all mean traits at metamorphosis occurred among atrazine treatments; higher atrazine exposure increased time to metamorphosis and decreased weight, SVL, and hematocrit. Nitrate treatments were not significantly different. Significant interaction tests between atrazine and nitrate occurred for weight and SVL at metamorphosis; the specific type of interaction varied among treatments. Assuming an additive mixture model, at low atrazine (40 microg/L), the addition of 37 mg/L nitrate produced SVL values less than expected (a synergistic effect) while the addition of 292 mg/L nitrate yielded SVL values greater than expected (an antagonistic effect). A similar response was noted for tadpoles in the 320-microg/L atrazine treatments. These results indicate that environmentally realistic concentrations of atrazine exert a negative impact on amphibian metamorphosis. Also, this study suggests that mixtures of agricultural chemicals, even if sublethal, may exert negative and not necessarily consistent mixture effects.

Two species of fishes common to the Great Plains of North America (the western mosquitofish [Gambusia affinis] and the sand shiner [Notropis ludibundus]) were used to examine the relationship between individual time-to-death (TTD) in 96-h toxicity tests and average fluctuating asymmetry (FA). Previous studies have indicated that FA, the random deviation from bilateral symmetry in metric traits, is increased in organisms that develop in stressful and/or marginal environments. This association has led to the prediction that FA may be useful as a bioindicator. For example, if an individual's FA is negatively related to stress resistance, a population that has comparatively high values of FA may be more susceptible to further stressors. Adult fish used in these experiments were captured from natural populations in Kansas and acclimated to laboratory conditions for 1 month before use in toxicity tests. Each species was separately exposed to two insecticides: parathion, an organophosphate, and lindane, an organochlorine. For each TTD test, 200 individuals were exposed over a 96-h period to an LC70 concentration. Dead individuals were removed every 3 h. Individuals that survived the test were sacrificed for comparison to those that died. Measurements for seven external morphologic characters were obtained for each individual and used to estimate average FA. Three of four statistical tests showed a significant negative regression between TTD and average FA, indicating that individuals with smaller FA survived longer in the test conditions. These results support the use of FA as an indicator of susceptibility to pesticides.

Abstract-Two species of fishes common to the Great Plains of North America (the western mosquitofish [Gambusia affinis] and the sand shiner [Notropis ludibundus]) were used to examine the relationship between individual time-to-death (TTD) in 96-h toxicity tests and average fluctuating asymmetry (FA). Previous studies have indicated that FA, the random deviation from bilateral symmetry in metric traits, is increased in organisms that develop in stressful and/or marginal environments. This association has led to the prediction that FA may be useful as a bioindicator. For example, if an individual's FA is negatively related to stress resistance, a population that has comparatively high values of FA may be more susceptible to further stressors. Adult fish used in these experiments were captured from natural populations in Kansas and acclimated to laboratory conditions for 1 month before use in toxicity tests. Each species was separately exposed to two insecticides: parathion, an organophosphate, and lindane, an organochlorine. For each TTD test, 200 individuals were exposed over a 96-h period to an LC70 concentration. Dead individuals were removed every 3 h. Individuals that survived the test were sacrificed for comparison to those that died. Measurements for seven external morphologic characters were obtained for each individual and used to estimate average FA. Three of four statistical tests showed a significant negative regression between TTD and average FA, indicating that individuals with smaller FA survived longer in the test conditions. These results support the use of FA as an indicator of susceptibility to pesticides.

Tadpoles of the African clawed frog (Xenopus laevis) were exposed to sublethal concentrations of atrazine (0, 40, and 320 microg/L) and nitrate (0, 37, and 292 mg/L) from feeding stage to metamorphosis. A 3 x 3 factorial design was used to identify both single and interactive effects. At metamorphosis, tadpole weight, snout-vent length (SVL), and hematocrit were determined. Mean mortality was greater in tanks receiving 320 microg/L atrazine; nitrate had no effect on mortality. Significant differences for all mean traits at metamorphosis occurred among atrazine treatments; higher atrazine exposure increased time to metamorphosis and decreased weight, SVL, and hematocrit. Nitrate treatments were not significantly different. Significant interaction tests between atrazine and nitrate occurred for weight and SVL at metamorphosis; the specific type of interaction varied among treatments. Assuming an additive mixture model, at low atrazine (40 microg/L), the addition of 37 mg/L nitrate produced SVL values less than expected (a synergistic effect) while the addition of 292 mg/L nitrate yielded SVL values greater than expected (an antagonistic effect). A similar response was noted for tadpoles in the 320-microg/L atrazine treatments. These results indicate that environmentally realistic concentrations of atrazine exert a negative impact on amphibian metamorphosis. Also, this study suggests that mixtures of agricultural chemicals, even if sublethal, may exert negative and not necessarily consistent mixture effects.

Differential genotypic tolerance of two species of fish native to the Great Plains of North America (the western mosquitofish [Gambusia affinis] and the sand shiner [Notropis ludibundus]) was examined in 96-h acute exposures to the pesticides lindane and parathion. Although a significant amount of data is available indicating that heavy metals cause selective genotypic mortality in aquatic organisms, very few studies have focused on the effects of pesticides on population genetic structure. Fish used in this experiment were captured from field populations in Kansas, USA, and acclimated to laboratory conditions for 1 month before the acute toxicity tests. Two hundred individuals of each species were exposed, in two separate experiments, to estimated LC70 concentrations of each pesticide for 96 h. Dead individuals were removed at 3-h intervals allowing for comparison of genotypic times-to-death. Individuals of each species were electrophoretically examined at two polymorphic loci. Phenotypes of mosquitofish were determined for phosphoglucose mutase (PGM ) and malic enzyme (ME ). Phenotypes of sand shiners were determined for PGM and aspartate aminotransferase (AAT ). Three of eight statistical tests (two species ϫ two loci ϫ two pesticides) indicated significant differences among survival curves of genotypic classes. Genotypes of ME and PGM of mosquitofish exhibited significantly different survivorship functions in the parathion and lindane time-to-death tests, respectively. Genotypes of AAT of sand shiners exhibited significantly different survivorship curves when exposed to lindane. Given certain experimental limitations, these results support a continued examination of population genetic characteristics as biomarkers of environmental contamination.