## 4 Occurrence of Micro and Nano-Plastics in Fish Gills and Consequent Effects  
The occurrence of microplastics in fish gills is reported in different studies on different fish species. Studies reporting NPs and MPs detection in gills refer to animals naturally exposed to these plastics [121; 122; 123; 124] and exposed to microplastics in laboratory condition [43; 69; 125; 126]. Like fish collected in fish farm and mariculture areas, those exposed to MPs in laboratory conditions show a higher or, at most, a comparable accumulation of MPs in gills than in gut [127; 128]. Therefore the MPs accumulation in gills could be influenced by microplastics shape [126], and the entity of accumulation in gills increases with increasing time of exposure [40] and is related to MPs size [129]. It is intriguing to notice that, once taken up, microplastics still exist in gill fishes after deprivation for seven days [130]. However, microplastics elimination time seems to be a specie-specific parameter as shown on larvae by [PERSON] et al. [131]. The effects of microplastics on gills (Table 2) were tested with in vitro and in vivo studies. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multicolumn{2}{c}{**Micro-and**} & \multirow{2}{*}{**Concentration**} & \multirow{2}{*}{**Exposition Time**} & \multirow{2}{*}{**Effects**} & \multirow{2}{*}{**References**} \\  & & & & & & \\  & & & & & & \\ \hline \multirow{8}{*}{\begin{tabular}{c} Japanese medaka \\ (_Orgzias latipes_) \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} 10,000 Microplastic \\ PP fiber \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} 10,000 Microplastic \\ fiber/L \\ \end{tabular} } & \multirow{8}{*}{\
\begin{tabular}{c} 21 days \\ \end{tabular} } & - & \multicolumn{1}{c}{increased mucus,} \\  & & & - & - & - \\  & & & - & - \\  & & & - & - \\  & & & - & - \\  & & & - & - \\  & & & - & - \\  & & & & - \\  & & & & - \\  & & & & - \\ \hline \multirow{8}{*}{
\begin{tabular}{c} Goldfish \\ (_Carassius auratus_) \\ \end{tabular} } & from 0.7 mm to 5.0 & 0.03 g of fiber/ 15 commercial food pellets; & & & \\  & between 2.5-3.0 mm & 0.96%, & & \\  & and 4.9/5.0 mm, & 1.36\%, 1.94\% and & & \\  & respectively & 3.81\% (g\) (food + \\  & fragments and pellets & MPs)/g www fish). & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Effects of micro and nano plastics on gills. [PERSON] et al. [136] found that polystyrenes NPs decreased the viability of rainbow trout gill epithelial cells in a concentration-dependent manner. On the other hand, [PERSON] et al. [132] conducted an in vivo study by exposing adult Japanese medaka (O. _latipes_) to polyester (PES) and polypropylene (PP) MPs fibers. By means of scanning electron microscopy (SEM), observations of gills increased mucus, denuding of epithelium on arches, and fusion of primary lamellae were observed. Histologic sections revealed: aneurysms in secondary lamellae, epithelial lifting, swellings of the inner opercular membrane that altered morphology of rostral gill lamellae. SEM and histochemical analyses showed increased mucous cells. Similar results were obtained by [PERSON] et al. [126]. They observed that microplastics fibers, retained in gills, cause the filaments fragmentation. According to the authors, this is probably due to the direct contact, and it is correlated to plastic particle size and shape. On zebrafish gills, MPs effects are similar to those induced on Japanese medaka, but [PERSON] and his team [138] revealed also a higher density of neutrophils in gill tissue of MPs-exposed fish. Microplastics effects on gills also depend on their exposure concentration. [PERSON] et al. [46] found that lipid oxidative damage in gills occurs after exposure to the higher MPs concentration they tested, but not to the lower. An oxidative stress status induced by microplastics was also detected on larvae gills after dietary exposures [140]. According to [PERSON] et al. [129], oxidative alterations were highly correlated with MPs size range.