The aquatic environment and the associated fish assemblages are being exposed to an increasing amount of microplastics. Despite the high number of publications on the presence of microplastics in fish, little is known about their uptake, translocation and accumulation within fish organs. Experimental studies on the detection and effects of pristine microplastics in fish have shown controversial and ambiguous results, respectively. Here, we conducted two experiments to detect and assess the impacts of dietary exposure of Danio rerio to different types of pristine microplastics. Our results show that D. rerio recognizes plastic particles as inedible materials but ingests them when mixed with food or fish oil. Accidental ingestion occurs in fish exposed to relatively small (1-5 µm) microplastic particles without associated food or fish oil. Additionally, D. rerio effectively eliminated pristine microplastics 24 h after ingestion; however, retention time was associated with increasing particle size and the intake of additional meals. Clinical signs, such as anorexia and lethargy, are present in fish fed relatively large microplastics (120-220 µm). The ingestion of microplastics does not induce any histopathological changes. To the best of our knowledge, we are able, for the first time, to fully demonstrate the uptake and translocation of plastic microbeads using confocal microscopy. Our results question the findings of previous studies on the detection and effects of pristine microplastics in fish and state that inaccurate interpretations of the histological findings regarding microplastics in fish organs is a prevalent flaw in the current scientific literature. The ever-growing production of plastics 1,2 and their relatively short lifespan 3 , combined with indiscriminate waste-disposal practices and accidental releases 1 , have led to the accumulation of plastics in aquatic environments worldwide 4-9. This situation is especially worrisome due to their long degradation time 2 and potential to be ingested by aquatic organisms 3. In water, plastics undergo weathering 10 through photolytic, mechanical and biological degradation 11,12. Under these circumstances, larger plastics degrade into smaller fragments 12 , i.e., microplastics (MPs) (< 5 mm) 13. Another source of MPs in aquatic environments is micro-sized particles intentionally manufactured for use in domestic products (e.g., cosmetics and clothing) and industrial products (media blasting and industrial feedstock) 11,12 , which are directly introduced into the environment by human activity 10. MPs can display a variety of shapes, sizes and colours 12. In aquatic systems, the predominant shapes of MPs are fibres 14-18 , fragments 6,7,19,20 and microbeads 21. The small size and ubiquity of MPs 22 makes them easily available to aquatic fauna, which are prone to ingest them by confusion with food, accidental ingestion or by transfer through the food chain 23,24. Several studies have documented the ingestion of plastic and MP particles in aquatic species (invertebrat...

Puffy skin disease (PSD) is an economically important syndrome in rainbow trout, Oncorhynchus mykiss (Walbaum, 1792). Because of its horizontal transmission, an infectious aetiology was hypothesised. However, aetiology remains unknown so far. PSD was first reported in 2002 in fish farms in England. Since then, cases have increased significantly, with additional reports in still water fisheries. Typically, the prevalence and severity of PSD increase during the summer and autumn months (13°C – 15°C). In February 2022, rainbow trout from a mountainous fish farm in Switzerland showed lethargy, inappetence, white to grey patches on the skin and slightly increased mortality. Twenty-one rainbow trout were submitted for diagnostics to the Institute for Fish and Wildlife Health, University of Bern, Switzerland. On post-mortem examination, 7/21 had up to 75% to 90% of the skin surface affected with bilateral, focally extensive white to grey, rough patches. The scales on the affected areas were raised, and the skin thickened. The histologic assessment revealed focally extensive, severe, epidermal hyperplasia with spongiosis and hydropic degeneration, severe scale pocket oedema with mild infiltration by degenerated neutrophils and lymphocytes, necrosis, and cellular debris displacing the scales. These findings are consistent with PSD. This is the first report of PSD in Switzerland. Here, contrary to previous reports, the disease occurred in rainbow trout exposed to 6-7°C water during the winter.

Trypanorhyncha are cestodes commonly infecting marine fish. Numerous studies have detailed the biology of Trypanorhyncha species, but information on the pathological changes produced by these parasites is limited. Dusky groupers are keystone species necessary for the preservation of several marine ecosystems. Considering their vulnerable state of conservation and the efforts being made to culture them, identification of the effects caused by Trypanorhyncha is vital. Here, we aimed to determine the prevalence and pathological changes produced by Trypanorhyncha in dusky groupers from the Canary Islands. The prevalence of trypanorhynch plerocerci was 96%. Grossly, in the abdominal cavity, there were numerous larvae-filled cysts and nodules. These were embedded in abundant fibrosis, producing visceral adhesions. Histologically, affecting the peritoneum, stomach, and intestine there were numerous degenerated encysted plerocerci and extensive deposition of mature connective tissue. These findings indicate that Trypanorhyncha is highly prevalent in adult dusky groupers from the Canary Islands, producing a progressive and chronic response. Furthermore, fish immune system appears to attempt to eliminate the parasites through fibrous encapsulation. Nonetheless, extensive fibrosis may have a detrimental impact on fish health when adjacent cells or tissues are compressed and their functions impaired.

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