## 3 Results and Discussion
### Abundance of Microl plastics Found in Mussel Digestive Glands  
Statistical differences in the number, size, and shape of MPs were found between sites (Table 1). The total number of identified MPs per mussel digestive gland is shown in Figure 5.  
The highest number of MP particles (32.85 \(\pm\) 27.98 MPs/digestive gland) was found in the digestive gland of mussels sampled from the ACI marina Rovinj (Croatia), with a significant difference when compared to Pula (\(p\) < 0.001), Katarina (\(p\) < 0.0001), Se\(\c \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Source of Variation** & **SS a** & **Df b** & **MS c ** & **F d** \\ \hline Total number & 8570.87 & 5 & 1714.17 & 7.76  \\ \hline Shape & & & & \\ Film & 44.37 & 5 & 8.87 & 4.69  \\ Line & 54.97 & 5 & 10.99 & 3.14  \\ Fragment & 2441.80 & 5 & 488.36 & 6.82  \\ Sphere & 3265.07 & 5 & 653.01 & 5.94  \\ \hline Size (\(\mu\)m) & & & & \\
5–10 & 3524.74 & 5 & 650.95 & 8.21  \\
11–15 & 280.66 & 5 & 56.13 & 3.81  \\
16–20 & 163.36 & 5 & 32.67 & 4.65  \\
21–25 & 10.96 & 5 & 2.19 & 0.90 \\ >25 & 112.87 & 5 & 22.57 & 2.13 \\ \hline \end{tabular}  
* SS—the sum of squares due to the source, b Df—degrees of freedom, c MS—the mean sum of squares due to the source, d F—the F-statistic, _*p_ < 0.0001, **_p_ < 0.01.  
\end{table} 
Table 1: Results of one-way ANOVA testing differences in total number, size, and type of MPs on mussel cryosections between sampling sites. Significant ANOVAs were followed by a Tukey post hoc test and, when relevant, \(p\) values (marked * or **) are given in the tables and text. Figure 5: Total number of MP particles in the digestive gland of mussels sampled at different locations: Katarina (native); Strunjan and Se\(\c\)a (cultured); and Kopar, Pula, and Rovinj (polluted—ACI marinas) in the northern Adriatic (N = 40 per sampling site). Data are reported as mean \(\pm\) standard deviation. Statistically significant differences between locations are marked with stars (*). (\(p<0.001\)) and Strunjan (\(p<0.0001\)). The location of ACI marina Rovinj was already previously noted as a highly polluted area [95; 96]. Thus, this result was awaited since this location is situated within the urbanized area of the city Rovinj, on the marine coast surrounded by a sea that is influenced by water originating from the fish processing factory \"Mirna\", wastewater from sewers, and ship docks. The following location, with slightly fewer MPs per digestive gland (21.30 \(\pm\) 19.71 MPs/digestive gland), was Kopar (Slovenia), a place surrounded by tourist settlements and, thus, influenced by industrial wastewater and the harbor. In the mussels sampled from the ACI marina Pula, 14.10 \(\pm\) 7.26 pieces of MPs were found per mussel digestive gland. The ACI marina Pula sampling site also accounted for a higher number of MPs per mussel digestive gland, indicating the influence of the immediate vicinity of a highly urbanized area, the port, and shipyards in Pula, which emit large amounts of urban and industrial waste into the water [95; 96]. Nevertheless, the number of MPs found in mussels from Pula was not statistically different from the number of MP particles recorded in the farmed mussels from Seca and Strunjan (Slovenia). These data are especially concerning since these mussels are cultured for human consumption. The lowest number of particles was noted, as anticipated, in the digestive glands of mussels sampled from the pristine sea, St. Catherine Island (8.35 \(\pm\) 5.59 MPs/digestive gland). The location at St. Catherine Island is located on the west coast of the Istrian peninsula and is 2 km away by airline from Rovinj. Therefore, this location was considered a control station since there were no sources of pollution close to the spot where the mussels were sampled. Nonetheless, it is still deeply concerning that even in this preserved location, acknowledged as a Natura 2000 site, MPs were identified in mussel tissue. This underscores the pervasive issue of MP pollution in the global seas.