Size-Based Ingestion of Microspheres and Microfibers by Two Freshwater Mussel Species (Dreissena bugensis and Elliptio complanata): Implications for Removal of Microplastic Particles from Aquatic Systems

Suspension-feeding bivalve molluscs encounter and selectively process microplastics (MP; ≤5 mm) in the natural environment. Ingestion and rejection of these particles will affect the potential of bivalves to be used to remove MP from aquatic systems. In this study, the size-based ingestion of multiple shapes and sizes of MP by two species of freshwater mussel was investigated. The quagga mussel, Dreissena bugensis, and eastern elliptio, Elliptio complanata, were delivered MP fibers and spheres ranging from 20 µm to 1 mm in size. Biodeposits (pseudofeces and feces) were collected to examine the number and proportions of each MP type that were either rejected or ingested by each species. Large microfibers and spheres (250 µm, 500 µm, 1 mm) were rejected in high proportions up to 100% by both mussels, whereas the smallest particles (20-µm spheres, 50- and 75-µm fibers) were rejected in proportions as low as 13.25%. The influence of size on the rejection of MP by these animals provides further evidence that suspension-feeding bivalves are ineffective bioindicators of MP pollution. The packaging of MP into biodeposits, however, presents a potential avenue for the removal of MP from aquatic systems and may have implications for the use of bivalves in engineered, nature-based removal systems.