The effects of ammonium perchlorate on survival, metamorphosis, and reproduction in the South African clawed frog, Xenopus laevis, and native amphibian populations
Goleman, Wanda Lynn Carney
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Xenopus laevis and native anuran embryos and larvae were exposed to ammonium perchlorate (AP) in a series of experiments. Five- and seventy-day median lethal concentrations (LC50S) for Xenopus laevis embryos were 510 ± 36 mg/L and 223 ±13 mg/L AP; 4- and 7-d LC50S for embryonic Rana utriculata were 15.6 mg/L and 3.12 mg/L. Exposure of X. laevis to sodium perchlorate (SP; or ammonium chloride (AC) showed AC to be significantly more lethal than SP, with 5-d LC50S of 98.3 mg ammonium/L and 3177.2 mg perchlorate/L. Although AC (14 mg/L) reduced snout-vent length, there was no effect observed on thyroid-sensitive indices of metamorphosis while similar concentrations of SP and AP inhibited metamorphosis. AP (14 mg/L) reduced whole-body thyroxine content and caused significant thyroid follicular epithelium hypertrophy in Xenopus larvae, both of which were reversed during a 28-d recovery period. AP exposure also skewed the sex ratio, reducing the percentage of males at metamorphosis. Rana catesbeiana tadpoles exposed to perchlorate (14 or 118 mg/L) for up to 264 h demonstrated a continual linear increase in whole-body uptake, with perchlorate content after 96 h of recovery no different from control animals. Individual tissue accumulation was as follows: thyroid (1072 mg/L) > whole-body (765 mg/L) > liver (725 mg/L) > kidney (551 mg/L) > stomach (447 mg/L). Utilizing the U.S. Environmental Protection Agency Endocrine Disrupter Screening and Testing Committee (EDSTAC) Tier I tail resorption assay, AP (14 mg/L) significantly inhibited tail resorption in X. laevis. A modified EDSTAC assay using surface waters collected from Longhorn Army Ammunition Plant (LHAAP) found no difference in tail length. These waters were found to have less perchlorate than previously reported, as well as a significant amount of iodide. A later study found that the simultaneous exposure of X. laevis to AP (14 mg/L) and sodium iodide (Nal; 14 mg/L) reversed the antimetamorphic effects of AP. Although AP was not teratogenic, concentrations reported in contaminated surface waters did inhibit metamorphosis. Even though these effects are negated by an adequate supply of iodide, the continued linear uptake of perchlorate, coupled with the relatively slow elimination, suggests that even episodic exposures have the potential to alter thyroid function in developing anurans.