A comparative study has been made of the toxicity, absorption and depuration of fenitrothion (dimethyl 3-methy1-4-nitrophenyl phosphorothionate; FS) and its oxon (FO) in Japanese tiger shrimp Penaeus japonicus. It has been previously reported that the toxicity of FO to the shrimp was approx. 10–20 times higher than that of FS in the intramuscular administration test, and also that FO showed ca. 12, 000 times high acetylcholinesterase inhibition compared with FS. In the lethal concentration (LC50) test, however, the toxicity of FO to young and juvenile tiger shrimp was only 1/10-1/50 that of FS, contrary to its toxicity in vivo. The results in the absorption and depuration test of [14C} FS and [14C] FO for tiger shrimp juvenile, demonstrated that the absorption rate of FS in the shrimp was approx. 40 times that of FO, and that the FS absorbed by the shrimp from surrounding water was rapidly biotransformed to more toxic FO, resulting in the high toxicity of FS to the shrimp compared with FO in the LC50 test.
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