## 1 Introduction  
Bivalves are globally distributed along the coasts of many countries inhabited by more than the half of world's population. Several features, such as sessile nature, filter-feeding, and bioaccumulation of trace metals in much higher concentrations in comparison with the ambient environment make bivalves suitable bioindicators of trace metal contamination [1]. During the last decade, the Mediterranean mussel _Mytilus galloprovicialis_ (L.) has been used in many trace metal studies [2; 3; 4; 5]. Moreover, _M. galloprovicialis_ L. was defined as a common indicator for trace/heavy metal pollution according to the United Nations' environmental program for the Mediterranean region [6].  
The adverse effects of trace metals on marine organisms and indirectly on humans via the consumption of seafood are well known [7; 8; 9]. The anthropogenic impacts on many coastal waters are becoming increasingly pronounced due to large amounts of effluents enriched with toxic trace metals [10]. Physicochemical and biogeochemical processes may produce the heterogeneous distribution of trace metals in different environmental matrices of the Adriatic estuaries [11]. The Boka Kotorska Bay, southern Adriatic Sea, records constant growth of maritime transport with higher numbers of cruise ship visits per year [12]. These large ships contribute to increased sediment re-suspension, which can lead to the transition of trace metals from sediments to the water column. So far, the investigation of trace metal content in the Boka Kotorska Bay in seawater, sediments, sea grasses, and bivalves [13; 14; 15] has been conducted.  
Bivalves' soft tissues, rather than shells, were more popular among researchers in metal bioaccumulation studies [16]. Despite less involvement, trace metal assessment in bivalves' shells was widely accepted and applied in aquatic ecosystem monitoring [17; 18; 19]. However, short-term laboratory treatments indicated that bivalve's shells could be less suitable for metal detection due to a slower accumulation rate in comparison with the soft tissues [20]. Although bivalves were recognized as good bioindicators, a more accurate methodology of using bivalves in trace metal biomonitoring should be developed [21]. To improve the methodology, trace metal assessment in different mediums/parts of bivalves, namely the calcite and aragonite shell layers separately, both calcareous shell layers without periostracum, and separated periostracum and byssus threads [22; 23; 24; 25; 26], was carried out.  
The periostracum is a thin yellow to brownish organic layer at the shell surface that envelops calcified parts, protects bivalves against dissolution in acidic water, prevents fouling by microorganisms, and tends to accumulate trace metals from the environment [27]. The surfaces of periostracum and deposited crystals are covered by a proteinaceous organic matrix [28]. Recently, the isolation procedure of organic matrices (_conchixes_) from the Mediterranean mussel has been published [29]. During the process, an intact shell organic matrix, including the periostracum, was obtained. Considering the limitations of the soft tissues and shells to a certain degree, our idea was to examine _conchix_ as a relatively novel medium to improve the methodology for trace metal assessment.  
Keeping in mind the above, the main aim of this study was to investigate the accumulation patterns of certain trace metals in the products of shell demineralization (_conchix_ and shell extract), soft tissues, and the shells of the Mediterranean mussel at three sites within the Boka Kotorska Bay, Montenegro for the marine ecosystem assessment.