## 2 Materials and Methods
### Observation and Validation of Microplastic  
The incursion of airborne fibers is a recurrent phenomenon in MP research [88]; thus, rigorous precautions should be taken while processing samples. As general control rules, work surfaces were cleaned with 99% ethanol, and laboratory cotton suits and nitrile gloves with no dust were worn when working to minimize contamination from the air. Moreover, seven procedural blanks (one for each cryosection and staining per sampling site) were analyzed together with the samples to detect contamination. The procedural blanks were completely free of any form of plastic contamination, as was seen in our previous study [83]. Slides and coverslips were cleaned and checked under the microscope for contamination with airborne fibers before use. To avoid any potential airborne contamination, contamination-free slides were left exposed to air during dissection. Airborne contamination was considered to be negligible.  
For the SEM, cryosection samples were left without a coverslip. The type of MP was examined under an SEM (SEM FEI FEG25 QUANTA, FEI Company, Hilsboro, OR, USA equipped with OXFORD PENTAFET EDS detector, Oxford Instruments, Abingdon, Oxfordshire, UK), and the images were taken with an optimized acceleration voltage of 3 kV and a detector working distance of about 10 mm. During the SEM observation of particle morphology, shape and size, the qualitative elemental composition of MPs was used to distinguish MPs measured with an EDS detector from non-microplastic samples, as the plastics are carbon-based and other materials are expected to be non-organic [85]. According to the chemical composition of the detected items, some MPs were verified and excluded from non-MPs. The MPs counted in all size fractions were used to quantify MP abundance for each mussel cryosection by visual characterization under light microscopy, followed by chemical characterization by using SEM-EDS analysis to compare visual and chemical assignations of MPs in parallels.  
MPs and non-MPs analyzed using a light microscope with a polarizer are shown in Figure 2 (marked with a black and grey arrow) and Figure 3a and b, respectively. Based on previous research, MP particles found in the mussels were categorized based on their size, color, and shape [88; 89]. A visual assessment was applied to identify the morphotypes of the MPs according to the physical characteristics of the particles [27]. The MPs were classified into four morphotypes: film, line, fragment, and spheres. Film was characterized as an isolated part of large plastic debris; line was defined as an MP with a slender and elongated appearance; spheres were round MPs that looked like a ball in shape; a fragment was characterized as an incomplete part that could not be classified as film, sphere, or line. When classifying MPs, particles smaller than 5 \(\upmu\)m were not considered (Figure 2, white arrow). Furthermore, larger particles that were not MPs (Figure 2, gray arrow, Figures 3a and 4b) or were not included in the tissue section (Figure 3b) were also excluded from the count and defined as fiber contamination. Additionally, also according to the previous research conducted by [PERSON] and [PERSON] [89], MP particles found in the mussel digestive glands were classified into five categories according to their size: 5-10 \(\upmu\)m, 11-15 \(\upmu\)m, 16-20 \(\upmu\)m, 21-25 \(\upmu\)m, and greater than 25 \(\upmu\)m. Further, MPs found on six different stations were classified into eight categories according to color, namely, yellow, green, transparent, pink, blue, purple, orange, and white according to [PERSON] et al. [90].