This study was conducted to examine the acute toxicity of the organophosphorus pesticide (OP) parathion on embryos and yolk-sac larvae of gilthead seabream (Sparus aurata), and to investigate the effects of this compound on cholinesterase and carboxylesterase activity of seabream larvae in the phase of endogenous feeding. The 72-h LC50 for yolk-sac larvae (0.523 mg L⁻¹) was about two-fold lower than the 48-h LC50 for embryos (1.005 mg L⁻¹). Parathion significantly inhibited the activity of ChE and CaE activity in yolk sac larvae but there were not significant differences in the sensitivity of both esterases to parathion as inferred by their 72-h IC50 values. Larvae exposed to parathion for 72 h showed a 70% inhibition of the whole body acetylcholinesterase at approximately the LC50.
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