Physical effects of chlorpyrifos on two stages of Xenopus laevis

The effects of a 96-h static exposure to chlorpyrifos were examined in two developmental stages of Xenopus laevis (premetamorph and metamorph). Measures of effect included body length, mass, and swimming ability. Most parameters indicate that metamorphs were more sensitive than premetamorphs. For larvae exposed as premetamorphs, body length and mass were significantly lower after exposure to 0.001 mg/L and 0.1 mg/L, respectively. Premetamorph swimming activity was increased by a concentration of 0.001 mg/L. Body length and swimming activity were the most sensitive endpoints in metamorphs, showing effects at 0.0001 mg/L. The corresponding body mass was not affected until exposure to 0.1 mg/L. Based on recent environmental data, chlorpyrifos has a 0.1-32.8% probability of exceeding a 96-h time-weighted average of 0.0001 mg/L in U.S. surface waters.