Plastic ingestion by other macro-invertebrates in the nature

Besides bivalves, many other macro-sized invertebrates are ingesting plastic in the wild. For example lugworms (Arenicola marina) collected from the beaches of French, Belgian and Dutch coasts in the North Sea were observed to contain 1.2 microplastic particles on average per gram of tissue excluding fibers. The particles found inside the animals were identified as low-density and high-density polyethylene and polystyrene. The highest recorded concentration was 11.3 particles per gram of tissue, but the 24 hours long depuration period decreased the amount of microplastics in the gut.

In the Clyde Sea, Scotland, 83 % of commercially valuable Norway lobsters (Nephrops norvegigus) have been observed to contain plastic in their stomachs. Some fragments of polyethylene bags were found, but the majority of plastics were composed of monofilament strands; moreover, 50 % of all the examined lobsters had a ball of tangled strands inside their stomachs. It is suggested, that the strands originated from fishing nets and ropes, since the area researched has a long history of commercial fishing. The high prevalence of aggregations of strands in the stomachs indicates that this type of plastic is hard to excrete. Interestingly, the large lobsters and recently moult individuals had fewer plastics in their stomachs compared to the individuals in the intermoult stages – later it has been confirmed that the microplastics can in fact be lost at ecdysis. In another study made around Scotland, the prevalence of plastic inside Norway lobsters was observed to be 67 %. In this study the plastic fibers were identified being mainly nylon and polypropylene, but also smaller amount of polyethylene and polyvinylchloride films were found.

A third (33.5 %) of gooseneck barnacles (Lepas spp.) rafting on floating litter in the North Pacific Subtropical Gyre was observed to contain plastic particles. The amount of ingested plastics ranged between 1 and 30 pieces per individual, but most of the individuals (52 %) had ingested ≥ 2 particles. The size of the ingested plastics varied between 0.6–6.7 mm and 99 % constituted of fragments and 1 % of monofilament line. The fibers were not count since the high possibility of airborne contamination. Polyethylene was the most common plastic type, but also polypropylene, polystyrene and unidentified plastics were found. In this study no blockages of the digestive tract were observed.

KNOWLEDGE CORNER: Invertebrates and plastic in the Baltic Sea

The capability of microplastic ingestion by invertebrates living in the northern Baltic Sea area has been studied in few laboratory experiments. All studied taxa, including gammarids (Gammarus spp.), mysid shrimps (Neomysis integer and Praunus flexuosus), polychaete worms (Marenzelleria spp.), amphipods (Monoporeia affinis), blue mussels (Mytilus trossulus) and Baltic clams (Macoma balthica), ingested 10 µm polystyrene spheres in a relatively low concentration (5 beads/ml). Feeding mode and behavior of the animals affected to the amount of microplastic ingestion: filter-feeding bivalves were most efficient feeders followed by gammarids and mysids, whereas deposit-feeding amphipods and polychaetes ingested the lowest number of microplastics. When considering larger secondary microplastic particles (200–300 µm), only the Baltic clam has been shown to ingest them. Particles as big were not found inside polychaetes (Marenzelleria spp.) or amphipods (M. affinis), so they are probably too large for these animals. The smaller the microplastic the more probable it gets ingested. As in the Baltic Sea it has been shown that majority of microplastic particles are below the 330 µm size limit of monitoring, it is probable that smaller plastics pose a greater risk to the animals.

Physical damage caused by microplastic ingestion

A variety invertebrates, such as scleractinian corals and sea cucumbers, have shown to ingest small plastic particles in laboratory settings. The effects of microplastic ingestion are however not yet fully understood: it was shown that plastics can be retained in the gut cavities of scleractinian corals for 24 hours, but it is unclear, whether their intake impaired further feeding or affected the growth of the corals. In a similar manner, it is not known if the plastic ingestion has adverse effects to the physiology or fitness of sea cucumbers. There are, however, some studies that have managed to monitor the impacts of microplastic ingestion to different invertebrate taxa.

Deposit-feeding lugworm (Arenicola marina) exposed to unplasticized polyvinylchloride particles (130 µm by mean diameter) mixed in the sediment 5 % by weight showed decreased energy reserves by up to 50 % in laboratory. The depleted energy reserves were suggested to arise from suppressed feeding activity, prolonged gut residence time and inflammatory response of microplastics. Lower feeding activity might also have decreased sediment-reworking and hence oxygen levels in the sediment, which could have affected the survival and diversity of the fauna living inside the sediment. In another laboratory study lugworms were exposed to 10 µm, 30 µm and 90 µm polystyrene spheres for two weeks, but the animals did not show any adverse effects even though the concentration of the microplastics were approximately a thousand times higher than in the nature.

Acute exposure tests made with freshwater amphipod (Hyalella azteca) have revealed that the amount of ingested microplastics, the decrease in growth and mortality attributed to ingestion is dependent on the microplastic concentration. The median lethal concentration (LC50) during 10-day long exposure to polyethylene particles was 4.64 x 104 /ml whereas in polypropylene fibers it was considerably lower with 71.4 particles/ml. Both ingested polyethylene particles (10–27 µm) and polypropylene fibers (20–75 µm) were completely egested from the gut, although the gut clearance time was longer for fibers. It is suggested that the longer residence time in the gut affected the amphipods more, since the fibers were also proved to reduce the growth more than polyethylene particles indicating deficiencies in nutrient uptake.

Polypropylene fibers have been observed having similar chronic effects to H. azteca: they were found to affect the mortality, growth and reproduction of these animals in even smaller concentrations. The mortality increased in the highest concentration of fibers (22.5 fibers/ml), but growth and reproduction was decreased already in the concentration of 5.6 fibers/ml. It is suggested, that the delay in reproduction and growth were caused by deficiencies in nutrient uptake.

Brazilian fiddler crabs (Uca rapax) have been shown to accumulate 180–250 µm polystyrene microplastics in their stomach, gills and hepatopancreas in experimental conditions. The mechanism of retention in the gills and hepatopancreas is still unclear since natural particles, such as sand grains and detritus, do not accumulate into these organs in a similar manner . The effects of accumulation were not examined in this study, but it is suggested that the presence of microplastics in these organs might disturb for example respiration.

In another study 0.5 µm polystyrene beads fed to blue mussels (Mytilus edulis) accumulated in the stomach, haemolymph, gills and ovary of the crab (Carcinus maenas) using these mussels as prey. However, this secondary ingestion of microplastics was not observed to affect the physical condition or behavior of the crab and the amount of beads were seen to decline over the time.

Harmful substances associated with ingested microplastics

A study made with Australian amphipods (Allorchestes compressa) demonstrated that microplastics isolated from facial cleaning products can act as a vector for POPs into marine organisms. The study compared the concentrations of PBDEs in amphipods after they had been exposed to them in the water with or without microplastics. Interestingly, microplastics in the water lowered the overall uptake of PBDEs compared to unabsorbed free chemicals, but when microplastics were present, a larger proportional uptake of higher-brominated congerners occurred compared to lower-brominated congeners. The study emphasizes the need to separate the pathways of harmful substances via bioconcentration trough skin and respiratory surfaces and via bioaccumulation through ingested material.