## 3 Results
### PAHs Concentration by Molecular Weight and by Season  
We compared the concentrations of LMW, MMW and HMW PAHs in Mediterranean mussel, Manila clam, common sole and red mullet, being that these species are the ones on which more records are available in this database. Summary statistics for each class of PAHs in each species are reported in Table 2. Statistics and \(p\)-values for each test are reported in Supplementary Materials (Tables S9-S12). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Species** & **PAHs Class 1** & \(\mathbf{C_{mean}}\)2 & \(\mathbf{C_{min}}\)3 & \(\mathbf{C_{max}}\)4 \\ \hline _Mytilus_ & LMW & 0.00942 & 0.00000 & 3.96000 \\ _Mgillovrovinicalis_ & MMW & 0.01068 & 0.00000 & 1.05960 \\  & HMW & 0.00570 & 0.00000 & 0.34500 \\ \hline _Ruditapes_ & LMW & 0.00137 & 0.00000 & 0.03770 \\ _Philippinarum_ & MMW & 0.00197 & 0.00041 & 0.00810 \\  & HMW & 0.00058 & 0.00000 & 0.00690 \\ \hline _Solea solea_ & LMW & 0.00079 & 0.00006 & 0.01000 \\  & MMW & 0.00189 & 0.00009 & 0.01037 \\  & HMW & 0.00914 & 0.00000 & 0.73500 \\ \hline \multirow{3}{*}{_Mullus barbutus_} & LMW & 0.01030 & 0.00011 & 0.09385 \\  & MMW & 0.00299 & 0.00042 & 0.01050 \\ \cline{1-1}  & HMW & 0.00090 & 0.00000 & 0.00539 \\ \hline \hline \end{tabular}  
* _PAHs class_—class of PAHs based on molecular weight, 2 \(\mathbf{C_{mean}}\)—mean concentration, 3 \(\mathbf{C_{min}}\)—minimum concentration, 4 \(\mathbf{C_{max}}\)—maximum concentration.  
\end{table} 
Table 2: Summary statistics on LMW (low molecular weight), MMW (middle molecular weight) and HMW (high molecular weight) PAHs in Mediterranean mussel, Manila clam, common sole and red mullet caught in the Adriatic, Ionian and Tyrrhenian Seas. Concentrations are in mg/kg wet weight. Figure 3: Bar plot showing the number of observations (x axis) for each marine species present in this database (y axis). The bars are arranged in ascending order of number of observations, by taxonomic class. On top of each bar, in brackets, is the number of sources from which the above observations were obtained. Each color corresponds to the taxonomic class of the species. Species codes are on the X-axis and are made of the first three letter of genus and species. The graph was realized using ggbreak library (version 0.0.7) in R [54]. In all four species, MMW PAHs show the highest mean concentration, followed by LMW PAHs and, finally, HMW PAHs, which show the lowest mean concentration (Figure 4). In all cases, the difference between the mean concentration of each PAHs class was statistically significant (\(p<0.05\)), except for the difference between MMW and LMW PAHs in red mullet. For each class of PAHs, we compared mean concentrations measured in cold and warm months in Mediterranean mussel, Manila clam, and red mullet, controlling for the effect of sampling depth and sampling year (Figure 5). We excluded common sole from the analysis because all samples for which sampling date was specified in the original sources were collected in autumn (October-December). Summary statistics on each variable included in the present analysis are reported in Table 3.  
In Mediterranean mussel, the Wilcoxon rank sum test reveals that both MMW- and HMW PAHs are present at significantly (\(p<0.05\)) higher concentrations in cold months (Table S13). Moreover, ANCOVA shows that the same trend is statistically significant, even after controlling for the sampling depth and sampling year (Table S14). In Manila clam, the Wilcoxon rank sum test reveals that all PAHs classes (i.e., LMW-, MMW- and HMW PAHs) are present at significantly higher concentrations in cold months (Table S15). However, as shown by ANCOVA, the above trend remains statistically significant after controlling for sampling year only in LMW- and MMW PAHs (Table S16). Data on sampling depth for Manila clam were not sufficient to use this variable as covariate in ANCOVA. Figure 4: PAHs concentration (mg/kg wet weight) by molecular weight in (**a**) Mediterranean mussel, (**b**) Manila clam, (**c**) common sole, and (**d**) red mullet.