1. Micro/nanoplastics pose a new environmental threat to aquatic ecosystems. The model organism Daphnia spp. has been used in many exposure studies to investigate the effect of micro/nanoplastics on organism fitness. However, owing to variation in experimental approaches, it is difficult to compare the findings. The aim of our study was to systematically review the research on the effect of micro/nanoplastics on Daphnia fitness, identify research gaps and offer recommendations for future studies. 2. We synthesised 121 studies and extracted data for numerous categories concerning study design, micro/nanoplastic characteristics and ecotoxicological endpoints. 32 studies were included in a meta-analysis on the effect of micro/nanoplastics on Daphnia reproduction. 3. Existing research exhibits several limitations. The majority of experiments have been conducted exclusively using Daphnia magna, neglecting other species and leading to an inherent bias in the representation of the broader Daphnia genus. Then, these studies have predominantly used a single genotype of Daphnia, disregarding potential clonal variation. In addition, most experiments investigated only a single Daphnia generation, although the limited number of multigenerational studies available suggest an increasing toxicity trend with subsequent generations, even if there was no impact on the F0 generation. 4. Regarding the types of plastics tested, the majority of studies focused on pristine, spherical microplastic particles, primarily composed of polystyrene, with particle sizes of <100 μm, and at concentrations >0.1 mg/L. This narrow focus limits the applicability of the findings to environmentally relevant scenarios, where micro/nanoplastics can take various shapes and composition, undergo aging and usually occur at lower concentrations than those used in the studies reviewed. 5. The primary Daphnia response variable assessed was mortality, followed by variations in reproductive traits or body size. The meta-analysis focusing on reproductive traits unveiled a consistent and adverse influence of micro/nanoplastics exposure on the production of offspring by Daphnia. 6. Future studies should test environmentally relevant concentrations of micro/nanoplastics, focus on smaller, lake-inhabiting Daphnia species, incorporate clonal variation and extend the scope to include multiple Daphnia generations. Moreover, recognising the complexity of real-world scenarios, we recommend integrating assessments of micro/nanoplastic effects with multiple stressors. By simulating such conditions, studies can yield insights that better mirror the complexities of ecological systems and provide a more accurate representation of the potential consequences of micro/nanoplastic pollution.
