Chronic dietary exposure to polystyrene microplastics in maturing Japanese medaka (Oryzias latipes)

Fish studies report consumption of microplastics (MPs) in the field, and concern exists over associated risks. However, laboratory studies with adult fish are scarce. In this study, outbred and see-through Japanese medaka (Oryzias latipes) were fed diets amended with 500, 1000, or 2000 μg/g 10 μm fluorescent spherical polystyrene microplastics (MPs) for 10 weeks during their maturation from juveniles to spawning adults. No behavioral changes, growth differences, or mortalities occurred. In vivo examinations and histologic sections showed no evidence of translocation of MPs from the gut to other internal organs. Mature females experienced dose-dependent decreases in egg number. Scanning electron microscopic examination of gills and gut revealed MPs in both areas. Swollen enterocytes were observed on apices of gut folds only in exposed fish. These were particularly apparent in foreguts of the high exposure group. Enterocytes with eroded brush borders were found in foregut of high and medium exposure groups. Increased mucus production, in long strands and sheets, was seen over primary and secondary lamellae of gills. Histological analysis showed alteration in buccal cavity, kidney, and spleen. Thickening and roughening of epithelium in headgut and pharynx and cellular alterations in spleen occurred. Head kidney was the primary site of alteration. Glomerulopathy and nephrogenesis were observed in exposed fish, increasing in severity with exposure level.