Trophic transfer of microplastics from mysids to fish greatly exceeds direct ingestion from the water column

Predators ingest microplastics directly from the environment and indirectly via trophic transfer, yet studies have not investigated the contribution of each pathway to microplastic ingestion in fish. We assessed the relative importance of the two exposure routes using mysids (Neomysis spp.) and a benthic fish (Myoxocephalus brandti) as a model prey-predator system. We first exposed the mysids to fluorescent polyethylene beads (27-32 μm) at concentrations of 200 and 2000 μg/L. We then exposed the fish to water containing the same concentrations of polyethylene beads or to nine mysids pre-exposed to polyethylene beads. We quantified the size and overall mass of polyethylene beads in mysids and in fish to assess polyethylene beads fragmentation by the mysids. Mysids ingested 2-3 more polyethylene beads from water containing the higher concentration, and fish ingested 3-11 times more polyethylene beads via trophic transfer than from the water column. The percentage of fragmented particles was higher in mysids and in fish fed bead-exposed mysids, suggesting that the mysids can fragment polyethylene beads. Our experiments demonstrate that trophic transfer is a major route of microplastic ingestion by fish and that prey such as mysids can fragment microplastics. Small particles can translocate from the digestive system into tissues and exert adverse physiological effects. Trophic transfer of microplastics may therefore pose more serious threats to organisms at higher trophic levels.