Polystyrene microplastic exposure reduces fecundity and delays development of the nematode Litoditis marina despite unaltered food absorption

Microplastics (MPs) pose a significant threat to marine benthic ecosystems. Using cryptic species PmIII of the marine nematode species complex of Litoditis marina as a model organism, we investigated the ingestion of differently sized polystyrene MPs (1- and 6-μm), their impact on carbon absorption from C-labeled bacteria, and the subsequent consequences for key life-history traits and population dynamics. Consistent with its buccal cavity size, Litoditis marina readily ingested 1-μm MPs, both in the presence and absence of a food source, but did not ingest the larger 6-μm particles. Crucially, short-term carbon absorption from bacteria remained unaffected by either MP size, even when the nematode gut was heavily loaded with 1-μm MPs. Despite this lack of nutritional impact, chronic exposure to 1-μm MPs significantly reduced fecundity, delayed embryonic and total development times, and decreased the abundance of eggs, juveniles, and adults, thereby compromising overall population maintenance. This disparity suggests that the adverse population-level effects are not driven by reduced nutrient uptake but likely by alternative mechanisms, such as increased energetic costs or direct reproductive stress. Our findings underscore the necessity of long-term, multiparameter mechanistic studies conducted at environmentally relevant pollutant concentrations to fully assess the ecological risks posed by microplastic pollution.