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In this study, the effects of zinc nanoparticles (ZnO-NPs) and ions (ZnCl2) on the mortality, growth, maturation, and cellular respiration of the earthworm Eisenia andrei were assessed. Earthworms were individually exposed for 98 days, starting from the juvenile stage, to soils contaminated with either ZnO-NPs or ZnCl2 (125, 250, 500 and 1000 mg Zn kg−1 dry weight (dw)). Exposure to the highest-concentration ionic treatments (500 and 1000 mg kg−1) caused 100% mortality, while for other treatments, mortality did not exceed 15% at the end of exposure. Compared to the control treatment, both 125–1000 mg kg−1 ZnO-NPs and 125 or 250 mg kg−1 ZnCl2 stimulated earthworm growth, which might be due to a hormetic effect. ZnO-NPs and ZnCl2 caused different responses at medium Zn concentrations (250 and 500 mg kg−1): earthworms exposed to ionic treatment at 250 mg kg−1 were characterized by a significantly lower growth constant, lower cellular respiration rate, later inflection point, and higher final body weight than those exposed to ZnO-NPs treatments at the same (250 mg kg−1) or twice as high (500 mg kg−1) nominal Zn concentrations. However, differences were not observed in all examined parameters between the studied forms when the highest-concentration ZnO-NPs treatment was compared with the lowest-concentration ionic treatment, which was likely due to the same levels of available Zn concentrations in those treatments. Overall, different growth and maturation strategies accompanied by pronounced differences in cellular respiration were adopted by earthworms exposed to low and medium levels of either ZnO-NPs or ZnCl2. [ABSTRACT FROM AUTHOR]