Microplastic dynamics vary within the same fish species in fresh and seawater: The University of Tokyo reveals findings

A research group including Project Researcher Hilda Mardiana Pratiwi, Research Associate Toshiyuki Takagi, Project Researcher Suhaila Rusni, and Professor Koji Inoue of the University of Tokyo's Atmosphere and Ocean Research Institute used larvae of Javanese medaka (Oryzias javanicus), a species adaptable to both seawater and freshwater, to compare the elimination process of ingested microplastics (MPs) in the two environments. They found that in seawater, MPs were expelled quickly but some remained in the intestine, whereas in freshwater, MPs were eliminated more slowly but were less likely to persist. The study also revealed that these differences were due to variations in the movement speed of gastrointestinal fluids and that MP elimination was even faster when food was present in the digestive tract. The findings are expected to contribute to unravelling harmful effects of MP and were published in the international journal Science of the Total Environment on January 15.

MP pollution has spread worldwide, and its effects on aquatic organisms are being studied from various perspectives. However, little attention has been given to the possibility that these effects may differ between seawater and freshwater environments. Last year, the research group studied MP ingestion by Javanese medaka larvae acclimatized to seawater and freshwater and found that more MPs were ingested in seawater.

In this study, the researchers compared the retention time of ingested MP particles in the body. Because MPs can have harmful effects, their retention duration is directly linked to their potential toxicity. The larvae that were kept in seawater (seawater group) and those acclimatized stepwise to freshwater (freshwater group) were exposed to 108 fluorescent polystyrene particles (1 µm diameter) per liter for 24 hours. They were then transferred to fresh seawater or freshwater, and the excreted particles in water or feces were counted over time up to 24 hours after transfer. The results showed that particles were eliminated more rapidly in the seawater group.

Using the larvae reared in seawater and freshwater containing a fluorescent substance, they examined gastrointestinal fluid movement and found that the gastrointestinal fluids moved more actively in seawater. The active gastrointestinal fluid movement was considered to accelerate particle egestion. MP particles were expelled even more quickly in larval fish that were fed brine shrimp larvae after particle ingestion than in those that remained unfed. This suggests that food movement through the gastrointestinal tract further accelerates MP elimination.

Meanwhile, after 24 hours, particles remained in the gastrointestinal tract in both groups, and a higher number of particles remained in the seawater group. Even after five days of observation, more particles were excreted in the seawater group than in the freshwater group, regardless of feeding status, throughout the observation period. Particles in feces were also detected for a longer duration in the seawater group. The longer duration of particle retention in seawater is partly explained by the fact that the larvae in seawater ingest more particles. Other possible contributors include differences in the microscopic anatomy of the gut between marine and freshwater fish.

The mechanism of body fluid regulation in fish is largely the same across species, except for sharks and rays. In marine fish, osmotic pressure differences cause water to be continuously drawn out through the body surface and gills. To compensate for this water loss, marine fish actively drink seawater. In contrast, freshwater fish do not need to drink actively, as water is continuously absorbed through their gills and body surface due to osmotic pressure differences. As a result, marine fish may be more vulnerable to MP pollution.

Inoue said, "Microplastic pollution in the marine environment has become a major problem. The results of this study showed that even the same species can be affected differently depending on differences in environmental conditions, reminding me of the importance of understanding the effects in conjunction with mechanisms underlying the physiological functions, environmental responses, and other contributing factors in the body of each organism. As the Javanese medaka is widely distributed in Southeast Asia, I hope that this study will provoke people across Asia to think about microplastic pollution in environments. This study was independently planned by the first author after she was inspired by a lecture on environmental adaptation when she was a graduate student."

Journal Information
Publication: Science of the Total Environment
Title: Osmoregulation affects elimination of microplastics in fish in freshwater and marine environments
DOI: 10.1016/j.scitotenv.2024.178293