## 1 Introduction  
Small pieces of plastic with sizes from 1 \(\upmu\)m up to 5 mm are categorized as microplastics (MPs) and constitute the most numerous debris reported in the marine environment [1, 2, 3, 4, 5]. Consequently, the contamination by these particulates poses a major risk to many aquatic organisms [6, 7, 8, 9]. Included as Descriptor 10 in the European Marine Strategy Framework Directive (MSFD), this anthropogenic contaminant directive has the goal of safeguarding the marine environment across Europe, with the main goal of achieving a Good Environmental Status (GES) by 2020 [10, 11]. MPs are generally categorized into primary or secondary types; however, it is worth noting that these definitions lack standardization [12]. Primary MPs are intentionally manufactured and include small spherical particles derived from their application in cosmetic and personal care products, sandblasting media, and pre-production pellets [13]. Secondary MPs, such as fibers, fragments, and pellets, are formed indirectly from the breakdown of larger plastic units due to environmental degradation by UV light, heat, wind, and waves [14, 15]. The origins of secondary MPs can come from the improper disposal of plastic litter, the shedding of fibers during regular usage, the washing of synthetic textiles, and the degradation of tires, road markings, and paints [16, 17, 18, 19]. MPs can also be classified based on other criteria such as color, stage of erosion, shape (fiber, film, spheres, or fragment), or polymer type (polyethylene, polystyrene, nylon, etc.) [20]. Once MPs enter the marine environment, they become available in the water column for ingestion by marine organisms. Scientific research into the presence of MPs in the marine environment is growing rapidly [21]. The presence of MPs in the marine biota and abiotic compartments of the sea presents a great threat to the marine environment on a global scale. Very few studies have documented the amount of MPs present in the waters and sediment on the coastline of the Adriatic Sea. A study has documented an average concentration of 406 \(\times\) 103 MPs/km\({}^{2}\) while monitoring the Trieste Bay in the northern Adriatic Sea [22]. Furthermore, the mean density of plastic debris isolated from the sediment of five beaches on the northern Adriatic coast was recorded to be 12.1 items/kg [23]. Among the detected MPs, fragments have turned out to be the most dominant form of MPs found in the sediment, while MPs of \(<\)5 mm size amounted to the 61% of the recorded plastic items [23]. In the northern Adriatic Sea in the Gulf of Venice, another study screened the MP pollution in the sediment and the results pointed out a density of 28.4 to 2250 items/kg of dry sediment, with fragments being the most found form of MPs [24]. On the other hand, [PERSON] et al. [25] have documented from 0.5 up to 1.0 MPs/kg of dry sediment along the coast of Slovenia northern Adriatic Sea, which was unexpectedly low. This could be due to the differences in the used methods or the years in which the surveys were conducted (taking into account also the different sampled areas). In the south of the Adriatic Sea along the beaches of Montenegro, 609 MPs/kg of dry sediment was recorded, and the most documented shape of MPs was filaments, followed by fragments and films [26]. Even though the sediment was mostly screened for the presence of MPs in the Adriatic Sea, some studies have searched for MPs in the marine biota, mostly in fishes [27; 28; 29] and crustaceans [30]. Bivalves serve as sentinel organisms utilized to evaluate environmental conditions, quality, and pollution levels in various regions, including the Mediterranean Sea, the North Sea, and the Baltic Sea [31; 32]. Among bivalve species, mussels thrive in both intertidal and subtidal zones, enduring conditions ranging from heavily polluted areas to pristine sites [33]. They have been widely used in marine monitoring initiatives, primarily due to their sessile lifestyle.