Size Selection by Zebra Mussels (Dreissena polymoropha) Exposed to Microplastic Fibers

Plastic production is increasing rapidly at a global scale and microplastics are widely distributed in aquatic ecosystems and food webs. As filter feeders, bivalves present entry points for microplastics into food webs and numerous descriptive and experimental studies have addressed microplastic size selection by bivalves. However, much less is known about size selection of microplastic fibers by bivalves. This represents a gap in understanding of how microplastics impact bivalves and food webs, particularly given the numerical dominance of microplastic fibers in freshwater and marine ecosystems and in bivalve tissue. To address this gap, we tested two hypotheses: (1) the net accumulation of polyester fibers in zebra mussels (Dreissena polymoropha) is dependent on fiber size and (2) zebra mussel size impacts the amount of plastic fibers accumulated. We conducted two experiments to test these hypotheses. In the first experiment, we exposed zebra mussels to pink polyester fibers (PPF) of four different lengths (0.25–2.0 mm), offered separately. In the second experiment, we exposed mussels with PPF of three different lengths (0.25–1.0 mm) in a mixture. Both hypotheses were supported. In each experiment, PPF of the smallest size (0.25 mm) disproportionately accumulated in the mussels. In addition, PPF accumulation of 0.25 and 0.50 mm PPF increased as zebra mussel shell length increased. Our results have implications for how microplastic fibers in water columns may impact bivalves and their food webs and demonstrate that size distribution should be considered when sampling bivalves for microplastics in ecosystems.