Reference: Crooks, N., Parker, H., & Pernetta, A. P. (2019). Brain food? Trophic transfer and tissue retention of microplastics by the velvet swimming crab (Necora puber). Journal of Experimental Marine Biology and Ecology, 519, 151187.
If a plastic bottle falls into the ocean, it does not float around forever. Instead, it is eventually broken down into smaller plastic particles, called microplastics. Oceans are full of microplastics that result from degradation of everyday use plastic products. Even though microplastics are less than a quarter of an inch big, they threaten marine animals by entering their diets.
Once eaten, microplastics remain in the digestive system of any living creature for an indefinite amount of time. They can cause internal injuries, release toxins, or even fool the stomach by creating a fake sense of fullness without providing any nutrients. This possible impact of consuming microplastics is exacerbated by the fact that marine animals do not even need to eat these particles directly for them to show up in their tissues. They can accumulate microplastics by eating organisms down the food chain who have already retained them in their own bodies.
Filter feeders, like mussels, directly retain microplastics by filtering plastic-contaminated water. Animals that eat those mussels retain the microplastics in their own tissues, passing them on again if eaten by other animals in a process called bioaccumulation.
Bioaccumulation of microplastics can be harmful. Some marine animals plastic particles in their reproductive organs have been found to have a harder time mating and procreating. If crabs, fishes, or oysters cannot make babies, then the survival of those species is at risk. Through bioaccumulation of plastic, this risk could be transferred to other species in the food chain as well.
To study bioaccumulation of microplastics in the body, a group of scientists from the University of Brighton in the United Kingdom turned to velvet swimming crabs. This crab species, nicknamed the devil crab for its belligerent nature and red eyes, feeds on fish and smaller prey like mussels, making it a convenient model for investigating bioaccumulation.
The Trojan mussels
The scientists obtained male velvet swimming crabs from a fishery off the British coast and secured mussels that would become vessels for delivering microplastics to the crabs. Once in the lab, the mussels were placed in water with tiny fluorescent microplastic beads that glow in green under a microscope.
In a little over an hour, the mussels had filtered the plastic beads from the water and retained them in their tissues. The crabs feasted on the “microplastics-spiked” mussels, and were put on a diet of plastic-free mussels. The researchers then sacrificed and dissected some of the crabs to search for the plastic particles in their tissues.
They isolated samples of crab tissues from different parts of their bodies and ran them through a machine called a flow cytometer, which measures how much fluorescence – or amount of colored light – was emitted from the tissue of interest. From this experiment, they calculated the amount of plastic in each type of tissue.
Plastic in the body
As soon as one hour after eating their microplastics-spiked mussels meal, the crabs had predictably retained the plastic particles in their stomachs. More surprisingly, these particles had also traveled to other organs. For the first time, microplastics were shown to reach the brain of a crustacean species.
Three weeks after eating the microplastics-spiked mussels, the remaining crabs still had the particles in their tissues. Over time, the amount of particles had only decreased in the gills and stomach – possibly because these organs have a high liquid turnover, which may have helped to wash them out. However, the amount of particles in the brains and testes had barely decreased; it was especially high in the testes, peaking at three weeks after the meal.
Food for thought
The University of Brighton team found that the plastic particles diffused from the crabs’ food to their tissues, where they could remain for a long time. This ability of microplastics to travel from food to different parts of the body is very concerning because it may impact many species in the food chain, including humans. Marine animals aren’t alone in their consumption of microplastics – humans have been found to injest plastic particles as well, in part by eating contaminated seafood. Along with many other marine creatures, velvet swimming crabs are seafood. By eating crabs that are loaded with microplastics, we could possibly end up with plastic particles in our own brains.
The good news is that we – or our food – are unlikely to consume mussels that are filled to the brim with microplastics, as was the case in this study. This is because the researchers placed the mussels in an extremely high concentration of plastic particles, far surpassing those in the oceans around the world. The bad news? By merely residing in tissues of the body, microplastics may have the power to harm their host for the rest of their lives.
We don’t know yet how microplastics in the brain of might affect behavior and well-being of any living creatures, but it is possible that these tiny particles may be very disruptive if they accumulate. As microplastics in the ocean are not going anywhere any time soon, this study is a somber reminder that plastic pollution is a threat not only to marine animals, but to humans as well.
I am a PhD candidate at Northeastern University in Boston. I study regeneration of the nervous system in water salamanders called axolotls. In my free time, I like to read science fiction, bake, go on walks around Boston, and dig up cool science articles.