Let us take a look at bivalves commonly seen at the dinner table. Bivalves are essential ingredients in various dishes, such as paella and vongole bianco. On the other hand, microplastic has turned up in bivalves from various oceans. Microplastic is defined as pieces of plastic that have broken up to less than 5mm in diameter.
Many bivalves are filter feeders. Filter feeding is a type of feeding when the organism opens its mouth and takes in seawater, along with whatever else happens to be there, while filtering out the food particles in the water. However, when they are taking in seawater, they are also mistakenly feeding on microplastic.
A research group in Canada scattered bivalves on a shore in British Colombia on Vancouver Island (Davidson & Dudas 2016). After 3 months, they collected the shells and removed the flesh with nitric acid, then examined the residue under a microscope. They found multiple colorful pieces of plastic. Almost all of these were fibers used as chemical fibers, which represent 90% of all plastics (Davidson & Dudas 2016)．
In a study on shells purchased in fish markets in America and Indonesia, plastic (fragments or fibers) was found in one third of the shells’ alimentary canals (Rochman et al. 2015).
According to research in Belgium, where mussels (Mytilus edulis) are eaten a lot, it can be assumed that Belgians eat an average of 122 shells per person each year, and on average 90 pieces of microplastic are contained in one shell, thus the estimated amount of microplastic ingested per person per year would be 11,000 pieces (Van Cauwenberghe & Janssen 2014).
When we eat asari clams and oysters, we eat the whole body of the shellfish, including the alimentary canal. Briefly, eating oysters and other shellfish means ingesting plastics.
What Happens to Shells that Accidentally Ingest Microplastics?
A research group in France has reported abnormal reproductive states for Pacific oysters that ingest microplastics (Sussarellu et al. 2016). The team fed microplastic (pieces of polystyrene 2μm and 6μm in size) to oysters for 2 months. Afterwards, they reported the number of ova and sperm had decreased and the swimming speed of sperm had become slower (Sussarellu et al. 2016).
Furthermore, the viability of a larva born to parents that have ingested plastic decreases drastically (Sussarellu et al. 2016). In standard experiments, we usually use far more plastic than would be the case in nature, however, in this research they gave a lesser concentration than is commonly found in nature. Despite this, the results were worse than expected.
At the moment, we still cannot clarify whether eating shellfish that have ingested plastic causes human health problems. And even if we stop eating shellfish, it would be difficult to prevent them from entering the human body because microplastics (especially microplastic fibers) exist literally everywhere.