To elucidate the cause of an abrupt increase in the toxicity of organophosphorus pesticides to tiger shrimp Penaeusjaponicus with the progress of larval stages, the toxicity of the pesticides to the shrimp larvae and the inhibition of larval acetylcholinesterase (AChE) by the pesticides, and the metabolism of fenitrothion(FS) in the larvae were studied at different stages, respectively. The toxicity of thiono-forms to the larvae increased abruptly with the progress of stages, whereas that of oxo-forms was unchanged from nauplius to postlarva. The susceptibility of larval AChE to each pesticide was almost unchanged in these stages. FS metabolic activity of zoea was very low, but the activity increased abruptly with their growth and resulted in the increase of fenitro-oxon which displays extremely high toxicity as compared with FS, i.e., the increase in the toxicity of thiono-forms to shrimp larvae with the progress of the stages must be due to the increase in the oxidative desulfuration activity.

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