Plastic ingestion by bivalves

Plastic ingestion by bivalves in the nature

Aquaculture is performed on the coasts in natural sea water and the animals cultured are hence exposed to all the components, including the microplastics and associated chemicals, in the ambient sea water. Bivalves are filter-feeders that filter large amounts of seawater to feed on the suspended algae. The blue mussel (Mytilus edulis) is one of the most popular shellfish species in seafood industry and it is consumed by humans around the globe. Numerous laboratory studies have demonstrated the intake of microplastic particles by the blue mussel and recently knowledge about their prevalence in both wild and cultured populations has also been growing.

In addition to the blue mussel, other bivalves have also observed to ingest microplastics. For example 75 % of the brown mussels (Perna perna) in Santos estuary, Brazil, and 33 % the Pacific oysters (Crassostrea gigas) on the east coast of United States are reported to contain small plastic particles. The amount of ingested microplastics varies between 0.6 and 178 partciles per individual, but it is hard to compare the results since different procedures, calculation methods and contamination levels.

Microplastics found inside different species of bivalves

*time that animals spent in a microplastic-free water to clear their gut

The microplastic load as well as its composition has been reported to vary greatly between the species, but whether the differences are caused by the species-specific traits or the environment is, however, not known. Majority of the microplastics ingested by mussels are fibers, but also fragments, films and pellets have been frequently found inside the bivalves. It is suggested that the high prevalence of certain types of synthetic fibers ingested by bivalves originates from the plastic rope used in fishing nets. In the cultured bilvalves syntehic fibers may be originated from the plastic lines where farmed mussels are grown or from the anti-predator netting. For example in Nova Scotia and British Columbia, Canada, farmed mussels contained more fibers in average compared to wild mussels. Even though fibers seem to be most abundant plastic type inside bivalves collected from the nature, their effects are yet poorly known.

The most common plastic types found inside the different commercial bivalve species in China were reported being polyethylene, polyethylene terephthalate and polyamide. The size of these particles varied between 5 µm to 5 mm, but the microplastics less than 250 µm were the most common. In other studies no categorizing by size or plastic type has been made.

Since bivalves are an important food source for many animals in the nature, they have a potential to transfer microplastics higher into the food web. The bioaccumulation of microplastics from bivalves to crabs (Carcinus maenas) has been observed in a laboratory, but no such transfer is observed in the wild.

KNOWLEDGE CORNER: Interactions of bivalves and microplastics in the Baltic Sea

Physical damage caused by microplastics

Exposure studies on the harmful effects of microplastics in bivalves are done almost explicitly on spherical plastic particles, even though they are sparse in the environment. In addition, the concentration of the particles is usually multiple times higher in these studies compared to the known conditions in nature.

In a laboratory study using high-density polyethylene powder (< 0–80 µm) the particles were found from the gills and digestive system of the blue mussel (Mytilus edulis). The mussels are hence exposed to the plastic particles through these two routes, as noticed also in other studies. The study also showed that small plastic particles were not only accumulating to the digestive tract but also dislocating to the tissues. After 3 hours of exposure, dislocated microplastics in the digestive gland caused inflammatory response in the cells around them. A similar response is usually associated with environmental pollution and indicates decreased health of bivalves. After 6 hours of exposure, lysosomal stability decreased, which is usually a sign of stress, toxicological response and associated pathological changes. The symptoms got worse with time until the experiment ended at 96 hours, but it is not known whether they were caused directly by the physical ingestion of plastics or their chemical composition.

Also in another laboratory study polystyrene beads of 3 µm and 9.6 µm have been shown to accumulate in the digestive tract of the blue mussel (M. edulis). Three days after the exposure the particles were seen to accumulate in their circulatory system and persisted there for 48 days. The accumulation of smaller particles was higher compared to the bigger size class. Even though the ingestion and translocation of these particles did not cause any measurable physiological effects, their persistence inside the bivalves for 48 days may increase the probability of plastics to get transferred to higher trophic levels.

Similar results have been obtained from the environment: a study examining live field collected bivalves have revealed that while the depuration time of blue mussel (M. edulis) is reported being typically 15 hours, even three days long depuration period is not sufficient to remove all the microplastics from the bivalves even though some purification was observed . This suggests that microplastics are able to accumulate in the bivalves on a more permanent basis. It is also been noted that the bigger microplastic particles are more efficiently removed from the digestive tract during the depuration of the gut compared to the smaller particles. Since small particles are demonstrated to dislocate from the digestive tract to the tissues  and to the circulatory system, it is proposed that small particles would be more persistent in the bivalves and therefore pose greater risk to the animals.

In addition to blue mussel, the effects of microplastic ingestion have also been studied using European flat oysters (Ostrea edulis). Oysters were exposed to high-density polyethylene particles with mean diameter of 102.6 µm and biodegradable polylactic acid particles with mean diameter of 65.6 µm in low (0.8 mg/L) and high (80 mg/L) concentrations. After two months of exposure the filtration and growth rates were not affected by the microplastics, but elevated respiration rates in oysters were observed with the higher dose of biodegradable plastic. However, the mean particle size for different plastic types was different, so the observed stress response might be caused by particle size rather than plastic type.

Harmful substances associated with plastics

The Mediterranean mussel (Mytilus galloprovincialis) was exposed to polyethylene and polystyrene beads < 100 µm for one week in a laboratory experiment studying the effects on virgin and pyrene-sorbed beads. Plastic beads were observed to transfer to mussels and accumulate in tissues, in particularly in digestive glands, where the concentration of pyrene appeared to be 3 folds higher than in plastic beads. In lower extent microplastics and pyrene were also found from the gills and haemolymph of the animals. Mussels exposed to virgin or contaminated microplastics caused similar effects indicating that immunological responses were mostly induced by the physical ingestion of the particles rather than their pyrene content. However, the genotoxic effect was higher in plastics with pyrene. It must be noted, that even though pyrene was present on the plastic beads in environmentally relevant concentration (200–260 ng/g), the abundance of microplastics was considerably high (1.5 g/L).