## 4 Occurrence of Micro and Nano-Plastics in Fish Gills and Consequent Effects
mykiss_[28], and did not present evident tissue damage in _Barbodes gonionotus_ exposed to PVC fragments (0.2, 0.5 and 1.0 mg/L) for 96 h [159]. Little is known about their uptake, translocation, and accumulation within fish organs [23]. MP/NPs showed that oxidative stress and its responding pathways, including inflammatory responses, could play the role of key events [132]. Several different types of effects have been observed in both freshwater [40; 82; 125; 160] and saltwater species [127] in the larval and adult stage. Polystyrene nanoparticles (PS NPs) were found accumulated in larvae or adult gut of Medaka (_O. melastigma_) with increased mortality and decrease in average lengths and weights [161]. Because of their microscopic size, micro- and nanoparticles could find in phagocytosis or endocytosis the preferred route of absorption. The occurrence of microplastics (<5 mm in diameter) and mesoplastics (5-20 mm in diameter) in the gastrointestinal tract of some commercially pelagic, demersal and reef fishes collected from Southeast coast of the Bay of Bengal was observed by [PERSON] et al. [162]. According to the custom of the Indian population to consume dry fish, the presence of microplastics in the fish gut is, in this case, a potential serious human health concern, as they are directly consumed. The transfer of microplastics of various types and sizes, through the intestine, to the lymphatic system has also been demonstrated. Some researchers have observed that the mechanism transporting MNPs through physiological barriers might be influenced by para-physiological conditions of the animal. In this regard, an increase in MPs transport in the colon of patients with inflammatory bowel disease related to an increase in intestinal permeability was observed [17], whereas a reduction of NPs transport in diabetes condition was seen [163]. NPs (100-1000 nm) stimulated the secretion of cytokines such as IL-6 and IL-8 from monocytes and they act as stressors to the innate immune system of fish [164]. Larger particles at about 100 \(\upmu\)m or above were proven to have no significant effect [28]. NPs can negatively affect fish at different stages of development, accumulating in tissues, decreasing locomotor and foraging activities. However, mortality, malformation or effects on hatching related to NPs have not been reported. High concentration of MPs/NPs are usually highly cytotoxic. ROS production and pro-inflammatory responses are the most frequently encountered. MPs/NPs toxicity is evaluated based on the response of gut lipid peroxidation biomarkers, as an indication of the response to oxidative stress, inflammation, epithelial barrier integrity and changes in gut microbiota [60; 61]. Therefore, the key role of exposure routes in the uptake, localization, and subsequent distribution of nanoparticles was highlighted by [PERSON] et al. [131]. The ingestion and effects of MPs in fish are largely dependent on their shape and size [126]. Toxicity of Micoplastics and Nanoplastics in Digestive Tract of Zebrafish: An Emerging Model to Study the Bioaccumulation and Toxicity of Environmental Contaminants  
Zebrafish represents an ideal animal model to study microplastic and nanoplastic toxicity. Due to the transparency of the eggs, embryos and post-hatch larvae, the localization of fluorescent-labelled MPs/NPs particles may be followed to determine the distribution, uptake, trafficking, degradation, and translocation using confocal microscopy [78; 165]. On Medline from 2015 to 2021, using key words \"microplastics\" and \"zebrafish\", we have n.78 available results, concerning the effects of micro/nanoplastics in different organs and tissues. Typing the keyword \"gut\" to get only 10 results. In general, the negative effect of microplastics on zebrafish was related to the concentration and particle size.