Acclimation to Cu in fathead minnows: Does age influence the response?

This study had two primary objectives. The first was to determine if the length of exposure necessary for acclimation to Cu to develop in larval fathead minnows (Pimephales promelas) is different than that for juveniles. The second objective was to determine whether the acclimatory response, as determined by organism survival, is consistent with acclimation as determined by whole-body Na⁺. Six experiments were conducted: four using larval (<20-d-old) and two using juvenile (<60-d-old) fathead minnows. Within each experiment, fish were allocated to one of four groups: unexposed, continuously exposed, episodically exposed or naïvely exposed. The continuous group was exposed to a sublethal Cu exposure (125 μg/L) for 8, 12, 16 or 20 d and then subjected to a survival test at a lethal dose (375 μg/L). Fish in the episodic group were exposed to the sublethal dose for either 4 or 8 d, given a depuration period of varying lengths (4-16 d) then subjected to a survival test. Naïve minnows were maintained in clean water then given the survival challenge. Results from survival tests show that the larvae acclimate after only a 4-d sublethal exposure to Cu. In contrast, juveniles require a 16-d exposure to acclimate. Once acclimation had developed, there was a strong relationship between larval survival and whole-body Na⁺. Acclimated larvae maintained whole-body Na⁺ relative to unexposed fish, while unacclimated larvae did not. Interestingly, this was not the case for juveniles, as acclimated and unacclimated groups did not differ with respect to whole-body Na⁺ concentrations. The results of this study show that age influences the time course and possibly the mechanisms of acclimation in fathead minnows exposed to Cu.