Comparing the effects and potencies of perchlorate and nitrate on amphibian metamorphosis using a modified amphibian metamorphosis assay (AMA)

Abstract

A modified amphibian metamorphosis assay was performed in which Nieuwkoop and Faber (NF) stage 47 Xenopus laevis larvae were exposed to different concentrations of either perchlorate (ClO4−) or nitrate (NO3−) for 32 days. Larvae were exposed to 0.0 (control), 5, 25, 125, 625, and 3125 μg/L ClO4−, or 0 (control), 23, 71, 217, 660, and 2000 mg/L NO3−. The primary endpoints were survival, hind limb length (HLL), forelimb emergence and development, developmental stage (including time to NF stage 62 [MT62]), thyroid histopathology, wet weight, and snout‐vent length (SVL). Developmental delay as evidenced by altered stage distribution and increased MT62, a higher degree of thyroid follicular cell hypertrophy, and an increase in the prevalence of follicular cell hyperplasia was observed at concentrations ≥125 μg/L ClO4−. The no observed effect concentration (NOEC) for developmental endpoints was 25.0 μg/L ClO4− and the NOEC for growth endpoints was 3125 μg/L ClO4−. Exposure to nitrate did not adversely affect MT62, but a decreasing trend in stage distribution and median developmental stage at ≥217 mg/L NO3− was observed. No histopathologic effects associated with nitrate exposure were observed. An increasing trend in SVL‐normalized HLL was observed at 2000 mg/L NO3−. Nitrate did not alter larval growth. The NOEC for developmental endpoints was 71 mg/L NO3−, and 2000 mg/L NO3− for growth endpoints. The present study provided additional evidence that the effects and potency of nitrate and perchlorate on metamorphosis and growth in X. laevis are considerably different.