ACUTE AND SUBCHRONIC TOXICITY OF METHYLENE-BLUE TO LARVAL FATHEAD MINNOWS (PIMEPHALES-PROMELAS) - IMPLICATIONS FOR AQUATIC TOXICITY TESTING

The aceta and 7-d subchronic toxicity of methylene blue were determine d for fathead minnow larvae using U.S. Environmental Protection Agency methods. The 96-h lethal concentration (LC50) was 45 and 15 mg/L at 2 0 and 25 degrees C, respectively. The 7-d chronic value for survival u nder laboratory light and low light was the same (2.1 mg/L). However, under laboratory light, larval growth was impaired at lower concentrat ions (chronic value = 0.6 mg/L) than under low light (chronic value = 2.1 mg/L). To assess prior methylene blue treatment on larval sensitiv ity, embryos, larvae <24 h old, and embryos followed by larvae were tr eated with 3 mg/L methylene blue. Cadmium LC50 values were similar bet ween methylene blue-treated and untreated control groups. When larvae treated with methylene blue as embryos were subchronically exposed to an effluent, survival was similar to that of the untreated group. Howe ver, dry weight of the treated larvae was significantly lower. Differe nces in dry weight between treated and untreated groups were reduced a s the age of the larvae at test initiation increased. Treatment of inc ubating embryos using 3 mg/L methylene blue does not appear to affect larvae sensitivity to cadmium but may affect the growth of larvae used in subchronic testing.