Is feeding type related with the concentrations of microplastics found in two marine invertebrates, the blue mussel (Mytilus edulis) and the brown crab (Cancer pagurus)?

05 Sep 2021

By Dr. Hazel Rooney

Take home message: Feeding behaviour contributes towards an organism’s vulnerability to microplastic ingestion, with microplastic pollution being prevalent among both scavengers (brown crabs) and filter feeders (blue mussels). However, the uptake of microplastics may be dependent on their shape and size, as well as the size of the organisms potentially ingesting them.

Introduction: Microplastics are commonly defined as plastic particles with an upper size limit of 1mm or 5mm. Their minimal size enables them to easily pass-through wastewater filtration systems and consequently, they are eventually discharged into large bodies of water. Microplastics have become ubiquitous in marine ecosystems, presenting a myriad of potential problems. The ingestion of microplastics has been demonstrated in laboratory exposure experiments, but the in situ conditions and impacts on individual organisms remain relatively unknown. Feeding behaviour is a factor that must be considered when evaluating the susceptibility of an organism to ingest and retain microplastics, due to the detrimental effect contamination may cause to marine trophic food webs. Therefore, the aim of this project was to investigate the ingestion and accumulation of two morphotypes (fragments and fibres) among two shellfish that exhibit contrasting feeding behaviours.

Experimental design: A total of nine blue mussels (Mytilus edulis), that exhibit filter feeding behaviour, and nine brown crabs (Cancer pagurus), that exhibit scavenging behaviour, were collected from the Northeast Atlantic Ocean. Once retrieved from the fishmongers, they were stored at -20℃ prior to analysis. They were assessed for the quantity of microplastics found in their stomach and gill tissues, through dissection and fluorescence microscopy.

Materials and Methods: Once thawed, the carapace width of each individual brown crab was measured and removed, followed by the removal of the cardiac stomach and the gills. Similarly, blue mussel samples were thawed, and the valve lengths measured before dissection. The mussels were then opened, and the gills and stomach of each mussel were removed. After each dissection, the stomach and gill set were rinsed, and each sample was filtered through Whatman Qualitative Filter Paper. Filter papers were then added to petri dishes, left to fully air dry and analysed under a fluorescence microscope. Two morphotypes of microplastics were counted separately from one another, fibres and fragments, and the number of fibres and fragments were counted in each specimen’s stomach and gills. All statistical analyses were carried out using RStudio for Windows and data was analysed using a two-way ANOVA (Species x Tissue Type) to assess the effect of species and tissue type on the number of fragments, fibres and total number of microplastics.

Results: Results showed that the effects of tissue type on the quantity of microplastics found were insignificant, meanwhile, the difference between the quantity of fragments, fibres and total microplastics found in the two species were all significant. The mussels were found to have a higher microplastic count, attributable to the remarkably vast number of fragments found. Inferring that organisms exhibiting filter feeding behaviour are more likely to ingest microplastics when in situ. The number of fibres found between the two species was higher for the brown crab, denoting that the ingestion of microplastics may be dependent on their shape and size.

This study was carried out by, Eleanor Stainer, while studying her for BSc in Conservation Biology at the University of Chester. Eleanor would like to thank her supervisor, Dr. James Brown, for his enthusiasm, guidance, and astonishing ability to respond to emails within 10 minutes throughout her studies. Eleanor is also thrilled that her dissertation has been highly commended by the BSAS! I wish Eleanor the very best of luck with studies and know that she has a very bright future ahead.

Dr. Hazel Rooney, Pig Technical Co-Ordinator, Alltech Ireland

Hazel has been a member of the BSAS Early Career Council since 2020. She works to help pig producers, feed mills and vets to improve the health, welfare, and productivity of pigs in the Irish and European marketplaces.