Cholinesterase activity and behavior in chlorpyrifos-exposed Rana sphenocephala tadpoles

Recent studies have found a correlation between organophosphate (OP) pesticide exposure and declines in amphibian populations. We evaluated the hypothesis that this relationship is driven by behavioral changes in developing larvae. Specifically, we examined how exposure to a common OP pesticide, chlorpyrifos, influenced cholinesterase (ChE) activity, mass, and swim speed in Rana sphenocephala tadpoles. We also determined how the presence of natural pond sediments in exposure chambers influenced response to the pesticide and how mass and survival were affected when tadpoles were exposed to an invertebrate (odonate) predator in addition to the pesticide. Mass and swim speed were measured after 4- and 12-d laboratory exposures to 1, 10, 100, and 200 microg/L of chlorpyrifos in test chambers that either did or did not contain pond sediments. These same parameters also were examined in mesocosms dosed with 200 microg/L of chlorpyrifos to evaluate responses under more environmentally realistic conditions. The effect of the invertebrate predators on survival and/or growth of tadpoles was evaluated in the mesocosm study and in separate laboratory experiments. In laboratory tests, no pesticide-induced mortality was observed; however, tadpole ChE activity in the two highest concentrations was significantly lowered, with a longer exposure duration further decreasing activity (maximum inhibition, 43%). Mass also was lower at higher concentrations, but this effect was not enhanced with longer duration of exposure. Reductions in ChE activity of tadpoles exposed in mesocosms were similar to those observed in laboratory experiments for the first 4 d. Tadpole swim speed and survival in the presence of a predator were not affected, with the latter largely resulting from pesticide-induced predator mortality.