Long-Term Tolerance Acquisition and Changes in Acetylcholinesterase Activity in Three Cladoceran Species After a 48-H Pulsed Exposure to Pirimicarb

We investigated (1) whether rapid acquisition of tolerance to pirimicarb would develop in three cladocerans ( Daphnia magna , Scapholeberis kingi , and Ceriodaphnia cornuta ) after short-term exposure and whether this tolerance was maintained in their descendants over four generations; (2) whether tolerance implies fitness costs, and whether these costs quantitatively correlated with tolerance levels; and (3) how AChE activity and AChE mRNA levels were altered by short-term exposure to pirimicarb. After 48 h of exposure to 0, 1.3, 2.5, 5.0, 10.0, 20.0, and 40.0 μg/L pirimicarb only in F0 generation, the surviving cladocerans in the concentrations (0, 1.3, 2.5, 5.0, 10.0 μg/L) were subsequently kept for additional three generations (F1, F2, and F3) in the absence of pirimicarb. Among the three tested cladocerans, the EC 50 value (50% effective concentration for 48 h exposure, using immobility as the endpoint) of only C. cornuta increased significantly. The increased tolerance in C. cornuta was retained in F1, F2, and F3. In C. cornuta , AChE activity and AChE mRNA levels in F0 decreased significantly, but these values in F1 were comparable to those in the controls, suggesting these changes may be related to the tolerance. Direct exposure of F0 C. cornuta to 2.5 μg/L or 5.0 μg/L pirimicarb induced a decrease in intrinsic population growth rate. However, this effect disappeared as soon as exposure was removed in F1. Thus, tolerance to pirimicarb, as observed in C. cornuta , involves no fitness cost. Our findings will contribute to clarifying adaptation of aquatic organisms to chemicals.