EFFECTS OF TEMPERATURE AND SALINITY ON THE TOXICITY OF A COMMERCIAL FORMULATION OF METHYL PARATHION TO CHASMAGNATHUS-GRANULATA (DECAPODA, GRAPSIDAE)

The toxic effects of a commercial formulation of methyl parathion on C hasmagnathus granulata (Decapoda, Grapsidae) submitted to different co mbinations of salinity(10 or 20 parts per thousand) and temperature (1 2, 20 or 30 degrees C) were studied. Static renewal tests were perform ed and the initial concentrations that killed 50% of the test organism s (LC(i)50) were estimated for each experimental condition (N = 60). O nly adult male crabs (mean weight, 9.46 +/- 0.05 g, and mean carapace width, 2.440 +/- 0.004 cm) were used. No significant differences were found (P>0.05) when comparing the LC(i)50 values for the two salinitie s at each temperature. Thus, a single LC(i)50 value was estimated for each temperature with the pooled data. The estimated LC(i)50 values we re 10.73 mg/l (8.75-13.87) at 12 degrees C, 2.16 mg/l (1.81-2.62) at 2 0 degrees C and 1.39 mg/l (1.10-1.64) at 30 degrees C. The values in b rackets represent 95% confidence interval limits. These data, which sh owed a positive temperature effect on toxicity, were significantly dif ferent (P<0.05) from each other. An hypoxic condition occurred due to branchial histopathological damage caused by the chemical formulation. This occurrence, combined with lower oxygen levels at higher temperat ures, may have been the cause of death and may explain the different L C(i)50 values observed. Another factor that should be taken into accou nt is the methyl parathion enzymatic activation by means of a mixed ox ygenase system, leading to the more toxic ore-form of the pesticide. W e suggest that higher temperatures preferentially favored activation r eactions over degradation ones, leading to increased toxicity of the f ormulation.