Plastics are ubiquitous in daily life, and global plastic production reached 359 million tons in 2018 (PlasticsEurope, 2019). Jambeck et al. (2015) estimated that 4.8-12.7 tons of plastics are released into the oceans each year. Numerous studies have reported the ingestion of plastics and their potential impacts on marine organisms such as whales, sea turtles, fish, crustaceans, soil fauna and soil microbe (Auta et al., 2017; Cole et al., 2013; Lazar and Gracan, 2011; Rummel et al., 2016; Zhou et al., 2020). Microplastics, defined as plastic pieces smaller than 5 mm in length, are an emerging threat to marine ecosystems (NOAA, 2020) because they are small enough to be ingested by even small organisms (Andrady, 2011; Cole et al.,2011) and difficult to remove from marine environments (Jambeck et al., 2015). Once ingested, microplastics can cause lacerations, inflammation, and starvation (Carbery et al., 2018).
Organisms ingest microplastics directly from their ambient environment or indirectly via trophic transfer (Walkinshaw et al.,2020). Most studies have focused on direct ingestion from the water column, and knowledge of trophic transfer of microplastics is scarce (Au et al., 2017), with the exception of a few reports that did not compare the contributions of direct and indirect ingestion (Farrell and Nelson, 2013; Nelms et al., 2018). Bioaccumulation studies using other environmental contaminants suggested that trophic transfer plays a more important role in the uptake than the waterborne exposure (Franklin et al., 2005; Kamunde et al., 2002; Qiao et al., 2000). Thus, trophic transfer may also be the major contributor to the ingestion for microplastics. Understanding the dynamics of microplastics in the marine food chain requires a quantitative comparison between direct ingestion and trophic transfer.
Although microplastics are mostly excreted after passing through the digestive tract, particles smaller than 10 μm can translocate from the gut into other tissues and cause adverse physiological effects, which have been shown in the blue mussel Mytilus edulis (Browne et al., 2008; Von Moos et al., 2012). Antarctic krill has been shown to fragment microplastics into particles small enough for tissue translocation to occur, perhaps through its feeding and digestion processes, which are shared by other small crustaceans such as copepods and mysids (Dawson et al., 2018; Kobusch, 1998; Michels and Gorb, 2015). If fragmentation of microplastics by small crustaceans is common, the particles may pose hazards to organisms at higher trophic levels.
The aim of the present study was to examine the relative ingestion of microplastics by fish from the water column and via trophic transfer from prey. We used a crustacean mysid (Neomysis spp.) and a benthic fish (Myoxocephalus brandti) as a model prey-predator system. We exposed the fish to fluorescent polyethylene beads (27-32 μm) and to mysids fed the beads to compare direct and indirect ingestion. We also analyzed the size of polyethylene beads ingested by the mysids and fish to assess microplastic fragmentation. We hypothesized the fish ingests more microplastics from the mysids than the water column and that the mysids fragment microplastics into smaller particles.
