## 4 Occurrence of Micro and Nano-Plastics in Fish Gills and Consequent Effects
The bioavailability of MP/NPs and consequently the effects at different levels depend mainly on their size, shape, concentration, and the presence of chemical pollutants [18,149]. The contaminants, species, developmental stage are considered as co-factors for plastic particle toxicity (PPT) [18]. As evidenced by numerous studies carried out under experimental conditions, the ingestion of microplastics can generate two different types of impacts on marine organisms: physical injuries to the organs where accumulation occurs and chemical damage by transfer and accumulation of pollutants with an enhancement of toxicant uptake or an increase in their bioavailability [17]. Conflicting results have been reported in literature regarding the effects of MPs and NPs accumulation in the gut. It has been demonstrated that the physical presence alone of MPs in the gut may be toxic due to their intrinsic ability to induce intestinal blockages or tissue abrasions, which may cause injury of the gut lining, morbidity, and mortality. Experiments carried out on _Dicentrachus labrax_ fed with microplastics for 90 days showed physical damage in the intestinal tract, both in specimens treated with contaminated plastics and in those treated with virgin microplastics [46]. [PERSON] et al. [146] reported that MPs might scratch gut tissues. While [PERSON] et al. [150] reported that MPs did not cause imminent harm in gilt-head seabream after 45 days of exposure, other studies reported abrasions, ulcers, false satiation, blockages in the digestive tract [151], dysbiosis in the gut [30,31], inflammation, bowel wall thinning, villi and epithelial damage, and oxidative stress in the gut tissues [152, 153, 154]. Histopathological changes in intestine were observed in goldfish (_Carassius auratus_) larvae by [PERSON] et al. [134] using PS MP/NPs of size < 5 um with negative repercussions on larval growth and elevated oxidative stress markers. A reduction in weight and body length was also observed in larvae of _Cyprinus carpio_ treated with PVC MPs [140]. MPs were found in gastrointestinal tract of wild fishes also, with higher lipid peroxidation levels in the different organs, including the gut [46]. Oxidative stress and structural damages by PS nanoparticles (10 um at 2-200 ug/L) accumulation in intestine were observed in adult _Oryza melastigma_[127] as well as in O. _latipes_ treated with PS MPs, 10 um. In this species, histopathological alterations with swollen enterocytes were observed [122]. In the literature there are also different opinions about the crossing of the intestinal wall by micro-nano plastics. Some authors claim that MPs in fish remain in the gastrointestinal tract and expelled with the feces [146,155]. Other authors found instead MPNs in fish fillets assuming rather that smallest plastic particles (<0.1 um) can transfer from the intestinal tract to the circulatory system and thus spread to the muscle as to all other organs and tissues [145]. In this regard, severe toxicity usually arise from smaller plastic particles with potential impacts on living organisms due to crossing biological barriers such as cell walls or intestinal barrier [145]. MPs intake during 90 days to _Sparus aurata_ induced an increase in oxidative damage and a pro-inflammatory response in gut of _S. aurata_, and were able to recover after the exposure to MPs was removed [156]. [PERSON] et al. [157] have estimated the uptake and gut retention of microplastics in juvenile stage of _Fundulus heteroclitus_ and red seabream, _Pagrus major_[157]. Various factors affect MPs bioavailability and level of risks at cellular and molecular level on aquatic organisms. Biomarkers for antioxidant response (superoxide dismutase, catalase, glutathione peroxidase, reductase, and glutathione S-transferase), neurotoxic impairment (acetylcholinesteraseterase), lysosomal activity alteration, and genotoxicity have been analyzed by [PERSON] et al. [158]. Virgin microplastics are not causing imminent harm to fish after dietary exposure in _S. aurata_[150] and _O.