Pesticides with chlorpyrifos and endosulfan as active element are used for pest control on agricultural lands and are high-risk inputs in aquatic systems. The acute toxicity of these insecticides in the freshwater prawn Palaemonetes argentinus was evaluated. The results were used to determine the lowest observed-effect and no observed-effect concentrations. Individual growth of prawns in relation to chlorpyrifos and endosulfan exposure was analyzed. LC(50) values to chlorpyrifos and endosulfan exposure were 2.98 microg L(-1) and 14.10 at 24 hours and 0.49 microg L(-1) and 6.28 microg L(-1) at 96 hours of exposure, respectively. The size increment of prawns was the same in all treatments; cephalothorax length increased linearly per molt. The intermolt period was influenced by the toxic effect of pesticides during rearing time, and this decreased with the molt cycles compared with the normal growth pattern. The results suggest that juveniles of P. argentinus are sensitive to chlorpyrifos and endosulfan pollution.

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