Microplastic pollution in annelids: A systematic review of species-specific impacts, toxicity pathways, and ecological risks

Microplastic (MP) pollution has emerged as a pervasive environmental stressor affecting terrestrial, freshwater, and marine ecosystems, with sediment-associated particles posing risks due to their persistence, bioavailability, and interactions with organisms and co-occurring contaminants. Annelids, including oligochaetes and polychaetes, are key ecosystem engineers that regulate sediment structure, nutrient cycling, and organic matter decomposition, making them highly relevant for evaluating the ecological risks of MP and nanoplastic (NP; <1 µm) contamination. Here, we synthesize peer-reviewed studies published up to 30 March 2025 to assess species-specific responses, toxicity pathways, and ecological implications of MP and NP exposure across terrestrial, freshwater, and marine annelids. By integrating evidence within a feeding-guild framework, we show that annelid responses are strongly trait-dependent rather than uniform. Deposit feeders and detritivores, such as Arenicola marina, Lumbriculus variegatus, Eisenia fetida, and Enchytraeus crypticus, frequently exhibit oxidative stress, altered energy allocation, gut microbiome disruption, and reproductive impairment. In contrast, filter feeders, including Sabella spallanzanii, often function as particle sinks with limited direct physiological effects, while carnivorous species, such as Hermodice carunculata, facilitate trophic transfer of MPs. Particle characteristics, particularly small size (<100 µm), irregular morphology, polymer type (notably polyethylene and polystyrene), and chemical aging, emerge as consistent determinants of adverse outcomes. Conversely, survival and population-level effects are often absent under short-term or environmentally realistic exposures, indicating compensatory physiological responses and limitations of standard toxicity endpoints. Overall, this review demonstrates that annelids act not only as sensitive sentinels of plastic contamination but also as active regulators of MP fate and ecological risk in sediment-based ecosystems.