## 3 Results and Discussion
### Size Distribution of Micoplastics Found in Mussel Digestive Glands  
Most of the MP particles found in the digestive glands of the mussel _M. galloprovincialis_ at all investigated locations fit the smallest category analyzed herein (5-10 \(\upmu\)m size), while the smallest number of particles were found to fit in the 21-25 \(\upmu\)m category (Figure 6). [PERSON] et al. have also demonstrated that mussels are more likely to ingest smaller-sized MPs (<5 \(\upmu\)m) rather than particles with greater diameter [75], which are less efficiently cleared than the larger ones (>10 \(\upmu\)m) [80]. This could become a problem for mussel predators, including humans, who could bioaccumulate and biomagnify MPs in their bodies through the marine food chain. [PERSON] et al. have proposed that the prevalence of MPs in a certain size range within mussel ingestion could be attributed to the abundance of those specific sizes in the water column [51]. Mussels are known to take up particles present in the water during their filtration process, which may explain the observed size distribution of ingested MPs. Furthermore, mussels can select particles for ingestion, and they often show a preference for smaller particles resembling their natural food items. These phenomena raise significant concerns about the environmental risks associated with smaller-sized plastic particles. Their ease of uptake by organisms (including mussels) and the increase in the bioavailability of associated chemicals amplify the potential risks and underscore the need for urgent attention and action to mitigate the impact of MPs on marine life and ecosystems. It was demonstrated in this study that mussels are prone to the ingestion of smaller-sized MPs into their digestive gland. These small size MPs can be further transported to other organs of the organism. Namely, in different studies, MPs were detected in the mussel foot, mantle, gills, and hemolymph [7; 52; 56]. Furthermore, several studies have observed the negative effects caused by the presence of MPs inside the mussel body. The abnormality in the mussel's body functions included DNA damage, oxidative stress, atrophy, and necrosis of the digestive gland tissue, reductions in condition and fitness indices, hypertrophy of the gills, decreased filtration rates, and alterations in the response of the mussel immune system and filtration processes [21; 63; 67; 76; 82]. Additionally, mussels that accumulate smaller-sized MPs in their tissue could pose a threat to higher predators throughout the marine food web [64]. The results obtained in our study point out that the highest abundance of the smallest MPs (size 5-10 \(\upmu\)m) occurred in the mussels sampled at the ACI marina Rovini location (19.20 \(\pm\) 17.30 MPs/digestive gland), while the same value was the lowest at the St. Catherine Island location (5.65 \(\pm\) 3.51 MPs/digestive gland). The number of MPs with a size of 5-10 \(\upmu\)m found in the mussels from ACI marina Rovini was significantly different from those found in Pula (\(p\) < 0.0001), Katarina (\(p\) < 0.0001), Seca (\(p\) < 0.0001), Strunjan (\(p\) < 0.0001), and Kopar (\(p\) < 0.05). Other size categories of MPs were found in smaller numbers and were inversely proportional to the size distribution up to the last size class (particle \(>\) 25 \(\upmu\)m), which showed no differences in sampled locations (Table 1, hence the graph is not shown herein). The location at St. Catherine Island had the lowest value of particles classified into the largest category (\(>\)25 \(\upmu\)m), while the other five stations had approximately similar values of these particles (4-7 MPs/digestive gland). Our described method using SEM allows for a highly specific differentiation based on size. Moreover, this method facilitates the further classification of very small MP particles, providing a comprehensive analysis of MPs in the sample. To date, mostly larger MP particles have been recorded in mussel tissue using different methods. In one study, the proportion of MPs in the blue mussel (_M.