## 4 Conclusions  
Overall, data from this study show that the exposure to virgin and CHR-loaded polystyrene MP did not alter the _M. galloprovincialis_ gamete fertilization and embryonic development, and generally induced qualitatively and quantitatively similar sub-lethal alterations in adult mussels. Single exposure to CHR were effective only at 0.1 mg/L CHR on embryological, lysosomal, antioxidant and neurological pathways; on the other hand, when exposing organisms to levels of freely dissolved CHR comparable to those sorbed on proffered MP, none of the abovementioned alterations were observed. Altogether, these results let us hypothesize that, at our experimental conditions, the levels of CHR sorbed on MP was not sufficient to induce cumulative and measurable effects on the tested species. Another possible explanation may be that sorbed CHR might not be readily bioavailable to \begin{table}
\begin{tabular}{c c c c c c} \hline
**Biomarkers** & **Toxicological Profile** & **MP** & **CHR-MP** & **CHR 0.1 mg/L** & **CHR 0.73 ng/L** \\ \hline _Cell level_ & & & & & \\ LMSGP & decreasing & AF & 0.71 * & 0.67 * & 0.35 * & 0.82 \\ LMSGP & decreasing & AL & - & - & - & NV \\  & & AF & 1.75 * & 1.64 * & 1.99 * & 1.03 \\ NL & bell-shaped & AL & + & + & + & NV \\  & & AF & 2.40 & 2.59 * & 2.93 * & 0.98 \\ LF & increasing & AL & NV & ++ & ++ & NV \\ GSTdg & bell-shaped & AF & 0.91 & 1.10 & 0.95 & 1.18 \\  & & AL & NV & NV & NV & NV \\ GSTg & bell-shaped & AF & 1.23 & 1.26 & 1.80 * & 1.01 \\  & & AL & NV & NV & + & NV \\ CATdg & bell-shaped & AF & 0.56 * & 0.82 & 0.77 * & 0.97 \\  & & AL & NV & NV & NV & NV \\ CATg & bell-shaped & AF & 0.95 & 0.92 & 0.80 & 1.03 \\  & & AL & NV & NV & NV & NV \\ AChE & decreasing & AF & 0.86 & 0.82 & 0.68 * & 1.19 \\  & & AL & NV & NV & - & NV \\ _Organism level_ & & & & & \\ Survival & decreasing & AF & 1.00 & 1.00 & 1.00 & 1.00 \\  & & AL & NV & NV & NV & NV \\  & & B & B & C & A \\  & & (Low stress) & (Low stress) & (Moderate stress) & (Healthy) \\ \hline \end{tabular} LMS—lysosomal membrane stability; NL—neutral lipid content; LF—lipofucsin content; GSTdg—glutathione S-transferase activity in digestive gland; GSTg—glutathione S-transferase activity in gills; CATdg—catalase activity in digestive gland; CATg—catalase activity in gills; AChE—acetylcholinesteraseare activity; GP—guide parameter; HSI—health status index; AF—alteration factor; AL—alteration level. AL thresholds for increasing/bell-shaped biomarkers: “NV” (no variation) = AF < 1.2; “++ = AF > 1.2; “++” = AF > 2.00; AF thresholds for decreasing biomarkers: “NV” = AF > 0.8; “-” = AF < 0.8;”-” = AF < 0.5. “-”, \(p<0.05\) vs. control.  
\end{table} 
Table 2: Output of the Mussel Expert System (MES) integrating biomarker responses of adult mussels exposed to MP, CHR-MP and CHR treatments in a unique health status index (HSI). mussels, which presuppose the involvement of adaptation or biotransformation processes of MP-sorbed POPs still not adequately characterized in marine organisms. Lysosomal endpoints were generally more consistently altered in response to selected treatments, confirming the role played by lysosomes as a main target of physical/chemical stressors in marine invertebrates. This investigation was conducted in controlled laboratory conditions using spherical PS MP to alleviate the likelihood of biases due to the co-occurring variables of physical and chemical nature. On the other hand, many factors may influence the sorption of POPs on plastic debris in a natural environment, including the presence of complex chemical mixtures, the chemical structure of dissolved chemicals and the size, structure and polymer type of MP. In this respect, data from this study may serve as baseline knowledge for future investigations using a more realistic environmental setting, including the sorption of chemicals in mixtures on micro/nano-sized particles of diverse nature.