By Justin Bauer
The use of durable plastic products has led to plastic items disintegrating into numerous small microplastics in the ocean. This poses a great threat to marine biota. Microplastic particles could damage digestive organs (Sonakowska, 2016) or accumulate and clog organs (Murray, 2011). Increased mortality rates due to microplastic ingestion have been observed in both Grass shrimp (Gray, 2017) and the Pacific mole crab (Horn, 2020). However, other studies have proven that other crustacean species are not affected by microplastics. To the contrary, the Atlantic ditch shrimp (Palaemon varians) can actively remove microplastic fibers from their stomachs through regurgitation. Understanding the adaptations of crustaceans to microplastic will be vital in ensuring their survival. Many shrimps are keystone species and are also consumed by humans, thus underscoring the importance of this topic.
The brown shrimp, Crangon crangon shows high reproduction rates and is the main target of coastal shrimp fisheries (Hünerlage, 2019). Their main food source consists of meio- and macrofauna species (Plagman, 1939), but along with that they ingest various inorganic particles, including microplastics. However, natural particles (from the sediment they live in) typically dominate microplastics in the shrimp stomach (Devriese, 2015). Once microparticles enter the digestive system of C. crangon a complex filter system will hinder particles larger than 1 μm from entering the midgut gland, the main site of nutrient resorption (Korez, 2020). Any particle smaller than that, will enter the midgut gland from where indigestible material is deposited into B-cell vacuoles and subsequently released through cell rapture into the lumen of the tubuli to be evacuated with the feces (Loizzi,1971). This mechanism most likely explains how C. crangon is able to deal with the high numbers of indigestible plastic particles in its digestive system.
The Palaemon varians species has adapted a different strategy. In an experiment, fluorescent microplastic fibers and microspheres were ingested by the shrimp with offered food. While the microbeads were passed further into the gut and then egested together with undigested food remains, the fibers were egested from the stomach through the esophagus in a process called regurgitation (Reinhard, 2019). However, if the shrimp received the food without the microplastics, regurgitation still took place. This behavior is most likely due to the P. varians’s herbivorous and detrivorous lifestyle in which many indigestible organic and inorganic particles need to be eliminated before injury or clogging of the digestive tract can occur. It seemed that the shrimp are selectively able to empty their stomach and this, in addition to the gastric filter separation of chyme and solids (Saborowski, 2015) indicates that regurgitation is an adaptation of the shrimp to expulse indigestible material.
However, the closely related species Palaemonetes pugio, was not able to rid itself of plastic fibers after exposure. In fact, after a three hour exposure to both 34 and 96 μm long fibers 100% of the 34 μm shrimp and 43% of the μm shrimp died (Gray, 2017). The Norway lobster, Nephrops norvegicus, was similarly unable to rid itself of plastic. The microplastic accumulated in its stomach, forming small balls, and clogging the stomach. While the balls were removed when the ectodermal structures of the stomach were shed during molting (Welden, 2016), the shrimp showed signs of starvation during their intermolt periods.
Since shrimp species are consumed by many other species, it is important to go on to evaluate the effects of plastic pollution on the nutritional quality of shrimp species. A study on White leg Shrimp (Litopenaeus vannamei) demonstrated that plastic contamination led to a decrease in essential amino acids and fatty acids (Chae, 2019). Similarly, a study on the Southwest Coast of India, proved that the commercially important Indian White Shrimp (Fenneropenaeus indicus) were contaminated with microplastic and any human consumption of those shrimp would subsequently lead to uptake of microplastics (Daniel, 2020).
Microplastic will have different effects on different shrimp species. Some will go extinct, some will adapt, and some will simply be unaffected. However, the extinction of some species will have massive effects on the ecosystem, and more research is needed to explore how we can rid our oceans of this miniature threat. If this is not a good enough reason to fund research, humans might be feeling the effects of biomagnification soon. While there has not been enough research done to investigate the effects of microplastic on the human body, it seems obvious that every attempt should be made to rid our ocean of microplastics.
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