Microplastics (MPs) are emerging contaminants in marine environments and represent a global issue that is no longer only discussed in the scientific community but has reached the level of public attention. Although MP ingestion has been reported for a wide variety of marine species, the toxicity to aquatic biota and the impacts on marine ecosystems are still poorly understood. In this study, organisms of two size classes of the estuarine bivalve Scrobicularia plana were exposed to a range of environmentally relevant polystyrene concentrations in acute toxicity bioassays. Polystyrene microparticles did not cause mortality nor did they alter the physiological state of the clams. Sub-lethal effects of microplastic exposure were assessed by a battery of biomarkers related to the antioxidant defense system and revealed that microplastic exposure led to alterations in antioxidant enzyme activities and increased oxidative damage. Results also indicated that effects were dose-dependent, with the most severe effects occurring in the highest tested microplastic concentration. The present work provides evidence of the adverse effects of polystyrene microparticles on S. plana and highlights the species’ role as a relevant target to assess the environmental threat of this contaminant.
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