Zinc is an essential metal for all living organisms. However, so far, little or no attention has been paid to the consequences of zinc deficiency or acclimation to this metal during culturing and testing on toxicity test results. In this study, the cladoceran Ceriodaphnia dubia was acclimated for 10 generations to four zinc concentrations ranging from 0 to 100 μg Zn/l and changes in zinc tolerance were monitored using acute (48 h) and chronic (9 days) assays. C. dubia deprived of zinc and acclimated to 13 μg Zn/l had a lower fitness in comparison with organisms acclimated to 50 and 100 μg Zn/l. In the two lowest versus the two highest acclimation concentrations the 9dEC50 values (on immobility) were 358–387 μg Zn/l versus 486–489 μg Zn/l; the mean number of young per female was 11–18 versus 25–32; and the time to first brood was 4.7–5.0 days versus 4.0–4.3 days. Moreover, the coefficient of variation of all parameters tested was highest in the two lowest acclimation concentrations. The results indicate that culturing test animals in media lacking trace metals such as zinc could give rise to animals that are unnaturally sensitive to those same metals during toxicity tests.
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