Abstract: Nanoplastics are able of causing adverse effects to marine organisms owing to their small size and large surface area to mass ratio. However, the presence of suspension additives in commercial nanoplastics widely employed in toxicological studies may introduce artifacts into nanoplastics toxicity assessments. In this study, the marine copepod Tigriopus japonicus was used as the test organism, and long-term exposure (24 d) was used to investigate the toxic effects of 50 nm amino-modified polystyrene nanospheres (PS-NH₂) and their filtrates (containing additive components) at environmentally relevant concentrations (5.5 μg/L, 55 μg/L and 550 μg/L) on the survival, growth, development and reproduction of T. japonicus. Our results revealed that both PS-NH₂ and their filtrates decreased the survival rate and body length of T. japonicus. At 550 μg/L PS-NH₂ exposure, the average molting interval and the development time to adult were delayed, while PS-NH₂ filtrates did not causethese toxicity effects. PS-NH₂ and their filtrates extended the hatching time, reduced number of nauplii. Compared to PS-NH₂, PS-NH₂ filtrates caused a higher rate of deciduous oocysts in female T. japonicus. In general, PS-NH₂ had a more adverse effect on survival and development than PS-NH₂ filtrates, and a less adverse effect on reproduction than PS-NH₂ filtrates, indicating a complex interaction pattern between PS-NH₂ nanospheres and additive components. Our findings contributed to the ecological risk assessment of nanoplastics in the marine environment.