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The effects of environmental contaminants on metamorphosis in Rana catesbeiana and sperm motility in Xenopus laevis Christensen, Jennie Rebecca

Abstract

The objectives of this research were to observe the effects of specific contaminants on different amphibian species at various times of their life cycles, namely metamorphosis and fertilization. The effects of a non-ionic surfactant, Nonylphenol (NP), on Rana catesbeiana (bullfrog) tadpole metamorphic staging and tail resorption were examined, along with the effects of pH, osmolality and a divalent metal ion, Zinc (Zn²⁺), on Xenopus laevis (African clawed frog) sperm motility and kinematics. I hypothesized that 1) NP would have indirect sub-lethal effects on thyroid hormone function by disrupting metamorphosis in bullfrog tadpoles, and 2) aquatic parameters (pH, osmolality and Zn²⁺ concentration) would have significant effects on percent sperm motility, velocities and kinematics. This latter hypothesis was the basis for the development of the Amphibian Sperm Inhibition Toxicological Test (ASITT) method. To test the effect of NP on metamorphosis, there were 3 NP concentrations used (234, 468 and 936 ug/L) in both the presence and absence of 3,3',5-triiodothyronine (T₃), a well water control, a T₃ control and a solvent control for a total of 9 treatments with three replicates each. Tadpoles were exposed for 7 days whereupon tail length, width, and metamorphic staging was measured. NP had a significant effect on tail length and metamorphic staging in the absence of T₃, causing an increase in tail length and a decrease in limb development, respectively, with increasing NP concentrations. In the presence of T₃, increasing NP caused a significant reduction in cranial transformation. These results suggest an indirect inhibition of T₃ during tadpole metamorphosis. For the development of ASITT, first the effects of pH and osmolality on sperm motility had to be determined. The pH 7.0 and 56.625 mosmol/L treatments gave the highest percent motility and the control solution for ASITT was then developed using these solution parameters. Zn²⁺ was used for preliminary validation of ASITT method. It was added to the control solution in concentrations from 0 to 1,417 μg/L for 7 treatments each with 12 replicates. Increasing Zn²⁺ concentrations caused a significant decrease in percent total motile sperm and progressive sperm. Straight line velocity increased with increasing Zn²⁺ concentrations. These results suggest that Zn²⁺ is inhibiting the sperm at early stages of motility, possibly by preventing calcium influxes into the cell or inhibiting cellular respiration.

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