Selective Ingestion and Egestion of Plastic Particles by the Blue Mussel (<i>Mytilus edulis</i>) and Eastern Oyster (<i>Crassostrea virginica</i>): Implications for Using Bivalves as Bioindicators of Microplastic Pollution

Microplastics (MP; 1 μm to 1 mm) of various shapes and compositions are ingested by numerous marine animals. Recently, proposals have been made to adopt bivalve molluscs as bioindicators of MP pollution. To serve as indicators of MP pollution, however, the proposed organisms should ingest, without bias, the majority of plastic particles to which they are exposed. To test this premise, eastern oysters, <i>Crassostrea virginica</i>, and blue mussels, <i>Mytilus edulis</i>, were offered variously sized polystyrene microspheres (diameters 19-1000 μm) and nylon microfibers (lengths 75-1075 × diameter 30 μm), and the proportion of each rejected in pseudofeces and egested in feces was determined. For both species, the proportion of microspheres rejected increased from ca. 10-30% for the smallest spheres to 98% for the largest spheres. A higher proportion of the largest microsphere was rejected compared with the longest microfiber, but similar proportions of microfibers were ingested regardless of length. Differential egestion of MP also occurred. As a result of particle selection, the number and types of MP found in the bivalve gut will depend upon the physical characteristics of the particles. Thus, bivalves will be poor bioindicators of MP pollution in the environment, and it is advised that other marine species be explored.