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The threat of plastic pollution in the ocean
These days, we hear more and more about the dangers of plastic pollution in the ocean. Every year, an amazing 4.8 to 12.7 million tons of plastic enter the world’s oceans. Plastics that enter the ocean break down in to smaller and smaller pieces over time and are generally characterized by size. Microplastics are tiny plastic pieces (smaller than 5 millimeters). For an excellent explanation on the prevalence and types of marine plastic pollution, see this recent post by envirobites author Mary Davis.
While there are ongoing efforts that attempt to clean up marine plastic pollution, such as The Ocean Cleanup Project, the reality is that marine creatures are encountering and even eating plastics on a daily basis. In fact, just this year two envirobites authors have published pieces on sea turtles (by Mary Davis) and microscopic animals called copepods (by Kari St. Laurent) eating plastic.
Microplastics come in a range of shapes and sizes including beads and fibers. Microfibers are found in textiles and account for more than 80% of the overall microplastics found in the Baltic Sea (Saborowski et al. 2019). Microfibers enter the coastal environment in high quantities because they are in the discharge of water from washing machines. In fact, it has been estimated that over 4 million microfibers are discharged daily by each municipal wastewater treatment plant in California, New York, and Wisconsin (Mason et al. 2016). Since microfibers and other microplastics are extremely prevalent in coastal marine environments, scientists in Germany recently tested the impacts of ingested plastics on the Atlantic ditch shrimp. To their surprise, they discovered the Atlantic ditch shrimp had evolved special mechanisms that it could use to get rid of accidentally ingested microfibers!
The tale of the Atlantic ditch shrimp
To determine whether Atlantic ditch shrimp accidentally ingested microplastics, Reinhard Saborowski and colleagues at the Alfred Wagner Institute in Germany mixed a solution of different sized microfibers and microplastic beads with shrimp food and placed it in aquaria with unsuspecting shrimp. After a few hours, they collected the shrimp and examined them under the microscope to look for microplastics in the stomach and guts. How could they see these? Well, Atlantic ditch shrimp are translucent (i.e. see-through) and the microplastics that they fed the shrimp were fluorescently dyed so they would glow under a specific setting on a microscope. The researchers found that the shrimp readily ingested microplastics of all different sizes, but only when the plastics were given to them with food. In treatments were shrimp were only fed plastic, very few microplastics were ingested. This indicated that the shrimp were accidentally eating the plastic when they were feeding.
Next, Saborowski and colleagues wanted to determine what happened to the microplastics that the shrimp accidentally ate. They conducted another experiment where they again fed shrimp the microplastic/shrimp food slurry, and then they placed the shrimp into a new aquarium with no food to track the shrimp over time. They wanted to know if the microplastics would pass through their digestive systems.
After tracking each shrimp with time-lapse recording, they were able to determine that the shrimp were able to rid themselves of all of the accidentally ingested microplastics using a combination of two mechanisms. First, small microplastic beads and fibers passed through the digestive system and ended up in the shrimp feces. But what about the microfibers that were too big to pass through? The shrimp were able to essentially puke these microfibers out, through regurgitation. The combination of these two mechanisms effectively eliminated all microplastic beads and microfibers from the shrimp and led to high survivorship in the experiment.
There are many groups of crustaceans and insects that are known to regurgitate, including some crabs and snails. While the exact triggers and control mechanisms for regurgitation are unknown, it appears to be a trait that has evolved to remove large indigestible food items from the stomach. Using this trait, the Atlantic ditch shrimp can rid itself of indigestible plastic and may possess a trait that will help it survive the age of microplastics. Future research should focus on the mechanisms underlying regurgitation and testing whether other species that possess this trait use it to keep plastic out of their stomachs.
Source Article: Reinhard Saborowski, Eva Paulischkis, and Lars Gutow. 2019. How to get rid of ingested microplastic fibers? A straigtforward spprach of the Atlantic ditch shrimp Palaemon varians. Environmental Pollution, Vol. 254B: 113068. https://doi.org/10.1016/j.envpol.2019.113068