## 4 Discussion  
Despite a significant correlation between the _conchix_ weight and dry shell weight, _conchix_ % in the shell showed a negative correlation with dry shell weight. Thus, the values of _conchix_ % in the shells of _M. galloprovincialis_ L. were relatively stable due to their reduced contribution to the overall increase in shell weight during the mussels' growth in comparison with calcified shell layers. This could be explained by the much lower content of organic (_conchix_) than inorganic material in the shells from this study. The concentrations of all metals analyzed were higher in the soft tissues and _conchixes_ in comparison with the extracts and shells. Only Mn content in the shells from Sv. Nedjelja (2.44 \(\mu\)g g\({}^{-1}\)) and Cogi (2.57 \(\mu\)g g\({}^{-1}\)) were higher than in _conchixes_ of mussels from the same locations (2.07 \(\mu\)g g\({}^{-1}\) and 1.6 \(\mu\)g g\({}^{-1}\), respectively). Mussels filter high amounts of water daily; therefore, they accumulate trace metals, which contribute to much higher content in the soft tissues than in ambient water [39]. However, despite different accumulation capacities, both soft tissues and shells can be used as bioindicators due to their ability to accumulate metals [40]. Trace metal concentration in the tissues is affected by the season, i.e., the reproductive cycle, as well as by environmental conditions, which can lead to higher concentrations of trace metals in winter and lower concentrations during the summer months [41]. It was explained by the dilution of metals during spring and summer due to the increased gonads' weight, which do not accumulate trace metals [42]. On the other hand, shells are considered more reliable indicators of contamination due to their lower trace metal variability in comparison with the soft tissues [43]. However, in our study, MPI values of organic mediums, such as soft tissues and _conchixes_, were >1, which indicated pollution, while the shells and shell extracts were unable to detect pollution (MPI < 1). The MPI values for the soft tissues at the investigated sites were in line with the data on the CI. A study by [PERSON] et al. [44] showed that trace metals were inversely proportional to the CI, which was not the case in our study. Higher contents of most elements in mussels' soft tissues could be expected due to the sampling period in late autumn long after the spawning period. The highest concentrations of Cu, Fe, and Hg were found in _conchixes_. The essential elements, including Fe and Cu, were more actively accumulated during the early ontogeny of the bivalve shells [45], indicating higher concentrations of these elements in the environment in the period of mussels' early life stages. Another study showed Fe distribution through the shell layers of _Arctica islandica_, with accumulation mainly in the periostracum [46]. Higher content of Fe in periostracum could contribute against dissolution by an acidic environment [47]. Moreover, higher Cu and Fe contents in _conchixes_ compared to tissues, as well as slightly higher Hg contents, could be attributed to the redistribution of  
Figure 5: (**a**) The PCA and (**b**) CCA plots of trace metal concentrations within the group of _M. galloprovincialis_ L. specimens sampled from Sv. Nedjelja (S), Cogi (C), and the IMB (I) in the shell (KS, KC, KI), soft tissue (TS, TC, TI), _conchix_ (OS, OC, OI), and shell extract (ES, EC, EI). these elements from soft tissues during detoxification processes [27]. Hg content in _conchix_ might be increased due to Hg variations during mussels' growth. The same processes can also explain site-related differences, i.e., highest Cu, Fe, Pb, and Hg concentrations in _conchixes_ from Sv. Nedjelja samples, while soft tissues from the same mussels had the lowest concentrations of Cu, Mn, and Pb. The contents of the rest of the analyzed elements were also higher in the soft tissues from one of the other sites, Cogi or the IMB. Moreover, none of the analyzed metals in shells and extracts were found in the highest contents in Sv.