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Contents: 1. Introduction 2. Global field investigations on microplastic pollution in mussels 2.1 Selected species and geographic coverage 2.2 Characteristics of microplastic pollution 2.3 Methodological challenges 3. Laboratory exposures of microplastics in mussels 3.1 Uptake, accumulation and clearance of microplastics 3.2 Toxic effects of microplastics 3.3 Optimization of laboratory exposures 4. Scope of mussels as global bioindicators of microplastic 4.1 Advantages of utilizing mussel 4.2 Current regional and national proposals 4.3 Future developments

This study documents the occurrence of microplastic (<5mm), mesoplastics (5-20mm) and macroplastic (>20mm) in Atlantic cod (Gadus morhua), a common and economically important species of marine fish in Norway. Fish stomachs (n=302) were examined from six different locations along the coast of Norway. Three percent of the individual stomachs contained items identified by Fourier transform infrared spectroscopy (FTIR) as synthetic polymers. Bergen City Harbour was a hotspot with 27% of the cod examined found to contain plastic. Polyester was the most frequently detected polymer. All bar one of the stomachs that contained plastic were full of organic stomach content, suggesting a plastic gut clearance rate similar to the ingested food. It is proposed that stomach fullness is an important metric in order to avoid underestimations when assessing the levels of microplastic ingested by fish.

Microplastic (MP) contamination is ubiquitous in the environment and many species worldwide have been shown to contain MP. The ecological impact of MP pollution is still unknown, thus there is an urgent need for more knowledge. One key task is to identify species suitable as sentinels for monitoring in key eco-compartments, such as coastal waters. In Norway, mussels (Mytilus spp.) have been monitored for hazardous contaminants through OSPAR since 1981. Norway has the longest coastline in Europe and adding MP to the Norwegian Mussel Watch is therefore important in a European and global context. The present study reports MP data in mussels (332 specimens) collected from multiple sites (n = 15) spanning the whole Norwegian coastline. MPs were detected at all locations, except at one site on the west coast. Among the most surprising findings, mussels from the Barents Sea coastline in the Finnmark region, contained significantly more MPs than mussels from most of the southern part of the country, despite the latter sites being located much closer to major urban areas. Only mussels from a site located very close to Oslo, the capital, contained levels similar to those observed in the remote site in Finnmark. In total an average of 1.5 (±2.3) particles ind and 0.97 (±2.61) particles w.w. g was found. The most common MPs were <1 mm in size, and fibres accounted for 83% of particles identified, although there was inter-site variability. Thirteen different polymeric groups were identified; cellulosic being the most common and black rubbery particles being the second. This study suggests Mytilus spp. are suitable for semi-quantitative and qualitatively monitoring of MPs in coastal waters. However, some uncertainties remain including mussel size as a confounding factor that may influence ingestion, the role of depuration and other fate related processes, and this call for further research.

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Mediterranean mussels (Mytilus galloprovincialis) were exposed over 21 days to polyethylene (PE) particles (0.01 mg ml; 50-570 μm) isolated from toothpaste. PE was deployed in the Outer Oslofjord (Norway) for 21 days, before exposing the mussels to both virgin (PE-V) and weathered PE (PE-W) particles. The mussels ingested both types of particles, but significantly more weathered particles were ingested than virgin (p = .0317), based on PE dosed by weight (mg ml) but not when considering particle number (PE-V: 1.18 ± 0.16 particles ml; PE-W 1.86 ± 0.66 particles ml). PE particle ingestion resulted in structural changes to the gills and digestive gland, as well as necrosis in other tissues such as the mantle. No differences were found regarding the degree of tissue alteration between PE-virgin and PE-weathered exposures. This current study illustrates the importance of using weathered particles in microplastic exposure studies to reflect the behaviour of plastic particles after entering the marine environment. The observed tissue alterations demonstrate the potential adverse effects to mussels exposed to microplastic particles.

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