An overview of the internalization and effects of microplastics and nanoplastics as pollutants of emerging concern in bivalves.

Microplastic and nanoplastic pollution in aquatic environments is a topic of emerging concern due to the internalization, retention time and effects of these particles in aquatic biota. Bivalves are considered bioindicators due to their wide distribution, sessile behaviour, occupation of ecological niches and ability to filter a large water volume. The study of microplastics and nanoplastics in bivalves has revealed the uptake mechanisms, internalization, distribution and depuration of these particles as well as their effects on physiological parameters, morphological alterations, immunotoxicity and changes in gene expression and proteomic profiles. In this review, we examine the primary characteristics of microplastics and nanoplastics (type of material, size, coating, density, additives and shapes) involved in their possible toxicity in bivalves. Furthermore, secondary characteristics such as the suspension media, aggregation stage and adsorption of persistent pollutants were also recorded to assess the impact of these materials on bivalves. Here, we have highlighted the efforts exerted thus far and the remaining gaps in understanding the extent of microplastic and nanoplastic impacts on bivalves on the basis of laboratory experiments and mesocosm bioassays and in the field. Furthermore, further microplastic and nanoplastic toxicological studies are proposed to facilitate the realistic assessment of environmental risk.