Plastic Pollution Drives Crabs Crabby

Primary Source: Vermeiren, Peter, et al. “Microplastic distribution among estuarine sedimentary habitats utilized by intertidal crabs.” Science of The Total Environment, vol. 866, 2023, p. 161400,

Ecological research in recent years has drawn great attention to the often overlooked importance of wetland ecosystems. From preventing floods to remediating agricultural runoff, wetland ecosystems provide a massive economic benefit to people, on top of being incredible hotspots for biodiversity. As many wetland ecosystems have been historically ignored or destroyed, protecting the ones we have left is a major ecological priority. However, because of these ecosystems’ close relationship with water, as well as their close position towards urban environments, recent concerns over the effects of microplastic pollutants have become increasingly important for wetland habitat protection. While wetland ecosystems may provide the same general benefits to people, there are various types of wetlands, with each having potentially different distributions and reactions to microplastics. With this in mind, researchers from Dutch and Japanese universities collaborated to study the distribution of microplastics throughout various sediment types and microenvironments within the wetlands, as well as how the different fauna of the wetlands may be affected by these pollutants.

An individual Chasmagnathus convexus, the larger of the crab species involved in this study. It mostly feeds on plant matter. Credit: “ハマガニ” by ふうけ, Wikimedia Commons, licensed under public domain.
Reeds Versus Mudflats

In this study, the researchers sought to measure the amount and type of microplastics found in the mudflats and the reeds of an estuarine environment, as well as how affected local animals living in these environments may be. This process was separated into two sections: assessing microplastic distribution among the sediment at each location, and assessing microplastic contamination of local crabs feeding in these areas, namely the species Chiromantes dehaani and Chasmagnathus convexus. Initial findings in the surface sediment found 2.6 times more microplastics in reed environments than in mudflats. However, at deeper sediment layers, the amount of microplastics were more similar between the two areas. This supports similar studies suggesting that vegetation may act as a trap for microplastics and fragmenting macroplastics. Seeing as surface environments did not significantly alter the amount of microplastics at lower depths, the next step in understanding how to approach the issue is to understand the relationship between microplastics and the sediment properties. By analyzing the microplastic content of sediment samples of varying properties, such as water content, grain size, and organic content, researchers were able to determine that microplastic content decreased with lower water content, higher grain size, and higher amounts of organic content. This indicates that, below the surface sediment, flowing water carries and spreads these microplastics throughout the sediment, between gaps in areas without sturdier, larger grain sediment or tough organic content.

Three individual Chiromantes dehaani crabs, the smaller species involved in this study, which are more prone to sifting through sediment to feed on microalgae. Credit: “クロベンケイガニ” by ISAKA Yoji (cory), Wikimedia Commons, licensed under CC BY-SA 2.1 JP.
Crab-full of Plastics

Aside from understanding the distribution of microplastics throughout the environment, understanding how these microplastics are affecting local fauna is also key to grasping the effects of microplastic on the ecosystem as a whole. To this end, researchers collected and immediately froze individual crabs of the species C. dehaani and C. convexus. The frozen crabs were then taken back to the lab, thawed, and washed of external debris to assure that any microplastics found were strictly from the crabs themselves. After dissection of the gut, internal contents were separated by density and sorted by material. Additionally, through isotope analysis of white muscle tissue, feeding habits between the two species were determined. They found that the larger species, C. convexus, fed nearly entirely on plant matter with small amounts of microalgae, whereas the smaller species, C. dehaani, fed on a more even mix of plant matter and microalgae scraped from sediment, with large amounts of sediment still present within the gut. Amount of microplastics between individuals varied greatly, however in both species there was at least one individual containing disproportionately high numbers of microplastics. Given that many of the plastics found in these individuals were of the same type, this suggests larger plastics are being shredded and ingested by these crabs, creating higher numbers of microplastics. This is especially true in the case of polystyrene foam. Theoretically, these smaller microplastics may then remain and possibly integrate into the body, causing adverse health effects depending on animal size and amount and type of microplastics retained. While this study did not find microplastics in high enough concentration to cause toxic effects, other considerations, such as the high variability between individuals and the tendency for toxic content to build in concentration up the food chain, make this an area where further study is needed.

Overall, this study provides insight into the ways microplastics spread throughout different parts of key ecosystems. Whereas much of the literature about microplastic pollution is still theoretical, this study observes actual distribution of micro and macro plastics, even showing that fauna like crabs can lead to the production of microplastics via feeding on and shredding macroplastics. This knowledge can help us protect these ecosystems of growing importance, letting us know which types of plastic may be more susceptible to shredding than others, or which get trapped in certain areas easier, and regulating those types of plastics accordingly.

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Cypress Novick

Cypress Novick

I am a recent graduate of Occidental College in Los Angeles, California, where I studied for my Bachelor's in Biology. My main research interests are wetlands ecology, mycology, estuary ecosystem interactions, and plant-based trophic interactions. I have always been passionate about making science more available and understandable, and am always trying to improve my writing so I may help myself and others be better understood.

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