Effects of microplastic pollution on fish health using Japanese Medaka as a model

Microplastic pollution is of current public concern for global environmental health and of relevance for aquaculture and fisheries. Microplastic ingestion has been shown to cause intestinal damage, microbiota dysbiosis, and disturbed lipid and energy metabolism in fish. However, these data resulted from exposures of adult fish at microplastic concentrations 2-7 times higher than concentrations found in the environment. To determine the impact of microplastic ingestion on fish health at environmentally relevant, chronic, low doses of microplastic fibers, Oryzias latipes larvae and juveniles were exposed to five concentrations of polyethylene (PE) fibers for 21 days through feed. Fish energy intake, growth & condition and reproductive output were assessed to determine the overall impact on fish health. To assess energy intake, gut integrity was evaluated at the molecular and cellular level. Relative gene expression of key digestive genes from gut tissue was quantified using real time-quantitative polymerase chain reaction (RT-qPCR). An effect on digestive gene expression potentially effecting nutrient absorption and antioxidant production was indicated via a significant decrease of slc6a6 expression in larvae exposed to the highest concentration. No significant molecular changes were observed in juvenile gut tissue. Gut morphology was analyzed using histomorphological observations of the intestinal mucus layer and cell types. No significant impairment to gut epithelial layers was observed in larvae or juveniles. To assess growth & condition, Fulton’s condition factor, hepatosomatic index, and gonadosomatic index were measured. No differences were observed in larval or juvenile growth. To assess reproductive output, exposed fish were bred and fecundity, fertility and hatching rate were evaluated. Fecundity and fertility were not affected by consumption of microplastic fibers at these concentrations. However, hatching in the F1 offspring was delayed by 2-3 days upon environmentally relevant PE exposure of juvenile fish. Significantly delayed hatching can impact vi overall offspring survivability and ultimately population recruitment. Comparisons of different developmental stages allowed for identification of vulnerable developmental stages for microplastic exposure; larvae were more susceptible to molecular changes due to PE exposure; however, juveniles demonstrated organismal level effects via delayed egg hatching. This study is one of the first to provide toxicological data on the risk of PE fiber ingestion during development stages of fish. Results indicate no imminent threat to fish condition at current measured environmental levels of microplastics; however, close monitoring of vital spawning grounds for commercially important fishes is recommended.