Characterization of salinity-enhanced toxicity of aldicarb to Japanese medaka: Sexual and developmental differences

The effects of salinity, gender, and development on the acute toxicity of aldicarb were examined in the euryhaline fish, Japanese medaka (Oryzias latipes). The 96-h median lethal concentrations (LC50s) at 1.5 parts per thousand (parts per thousand) salinity were not significantly different between adults and juveniles but larvae were significantly more sensitive to aldicarb. A two-week exposure to increased salinity significantly enhanced the toxicity of 0.5 ppm aldicarb to both sexually mature male and female medaka. After 48 h of aldicarb exposure, mortality significantly increased (p < 0.05) in males from 13 +/- 5.7% at 1.5 parts per thousand salinity to 56 +/- 5.7% at 20 parts per thousand; in females mortality significantly increased (p < 0.01) from 17 +/- 5.7% to 76 +/- 5.6%. A time-course study was conducted in which muscle acetylcholinesterase (AChE) inhibition was monitored after exposure to aldicarb. In general, AChE in females was inhibited to a greater degree at 12.0 and 20.0 parts per thousand salinity regimens than AChE in males. Muscle AChE in females residing at 20.0 parts per thousand was inhibited 93 +/- 3.3% by 8 h of exposure to 0.95 ppm aldicarb, whereas in males the maximum inhibition was 80 +/- 7.4% after 8 h of exposure to 0.86 ppm aldicarb at 20 parts per thousand salinity. These results indicate that environmental factors, such as salinity, in addition to gender and development, have significant impacts on the acute toxicity of aldicarb to Japanese medaka.