Stress induction in Hippolyte inermis Leach (1816) larvae by microplastics from manufactured polylactic acid and polypropylene cups

Crustaceans play a key role in aquatic food webs and may facilitate the transfer of microplastics (MPs) across trophic levels. The decapod Hippolyte inermis represents a sensitive model organism for assessing ecotoxicological responses to environmental stressors. In this study, planktonic larvae of H. inermis were exposed for ten days to three concentrations (1, 5, and 10 mg L⁻¹) of MPs derived from commercial polypropylene (PP) and polylactic acid (PLA) cups. Larval mortality and growth were evaluated, and molecular responses were assessed through the expression analysis of eighteen genes involved in stress and detoxification pathways. No significant differences in mortality or growth were observed between treated groups and controls for either polymer at any concentration. In contrast, all analyzed genes exhibited altered expression following MP exposure, with a predominant upregulation of genes related to stress response (e.g., Hsp70, p53, TNF) and detoxification processes (e.g., CytP450, ALDH), particularly at the highest concentration. These results indicate that, despite the absence of significant organism-level effects on mortality and growth, microplastics derived from commercial PP and PLAmcups induce molecular responses in H. inermis larvae, with the modulation of genes involved in stress response and detoxification pathways. Overall, the findings suggest that sublethal effects may occur at the molecular level even in the absence of overt phenotypic effects. The observed responses may be partially influenced by chemical additives associated with the tested materials, although this cannot be directly confirmed. These results highlight the usefulness of integrating molecular biomarkers in ecotoxicological assessments and suggest the need for further evaluation of the environmental relevance of commercial microplastics.