## 4 Discussion  
The development of this database on PAHs in seafood caught in the Western and Central Mediterranean Sea showed that most of the studies carried out from 1990 to 2021 focused on the Tyrrhenian and Adriatic Seas, with fewer records coming from the Ionian Sea. Moreover, Mediterranean mussel was by far the most studied species, accounting for more than half of records in the database. This is not surprising, given that, along with red mullet, Mediterranean mussel is considered one of the most suitable organisms to be used in biomonitoring studies, because of the widespread distribution, the ability to accumulate contaminants to a degree proportional to their bioavailability, as well as the ease of sampling [41, 56]. In this database, MMW PAHs and LMW PAHs showed higher concentrations than HMW PAHs in Mediterranean mussel, Manila clam, common sole and red mullet. This is in line with the expectations, given the greater solubility and bioavailability of lighter PAHs and the faster metabolism of the heavier ones [12]. In a recent investigation carried out using large-scale monitoring data on PAHs in sediments of the Mediterranean Sea [57], the authors found that the two most prevalent PAHs in the Western Mediterranean basin were fluoranthene (Flu) and phenanthrene (Phe), with the former being the most abundant also in the Adriatic Sea and in the Central Mediterranean basin. In view of this, it is interesting to note that, in our meta-analysis, Flu and Phe are also the most abundant PAHs in both Mediterranean mussel and Manila clam (Figures S5 and S6), which are benthic filter-feeding bivalves, and so may be particularly prone to absorbing PAHs accumulated in bottom sediments, after their remobilization. The analysis on seasonality shows that PAH concentrations tend to be higher in specimens sampled in cold months (October-March), and this trend is statistically significant in both Mediterranean mussel and Manila clam, confirming what several studies, including those in this database, found independently [8, 16, 19, 29, 32, 41]. As suggested by previous investigators, the reason for such a seasonal pattern may lie both in the biology of these species and in changes in the emission and mobilization of PAHs in the environment. PAHs are lipophilic compounds [49], and as such, they accumulate preferentially in lipid-rich tissues [50]. Given that the lipid content of tissues of marine species can vary under the effect of, for example, nutritional [13] and reproductive  
Figure 6: Boxplots showing the THQs distribution in several marine species included in this database. Species codes are on the X-axis and are made of the first three letter of genus and species. Boxplots are in ascending order from left to right, based on median values; outliers not shown. status [35, 58], one might speculate that seasonal fluctuations in such parameters may be reflected in PAH concentration changes. However, this should not be the case with Mediterranean mussel, since several investigations carried out in the Mediterranean Sea found higher lipid content in mussel sampled during summer [59, 60], and this is likely due to the depletion of lipids that takes place after spawning [58, 60]. Another possibility is that higher PAHs concentrations in winter are the result of increased PAH emission through, for example, domestic heat [12, 17, 61, 62] and industrial activities [63]. Moreover, in summer, the degradation of PAHs, especially of the lighter ones (i.e., LMW PAHs) [63, 64], is enhanced by UV radiation, high temperatures, and ozone [65]. On the other hand, low seawater temperature in winter can inhibit the microbial degradation of PAHs [66, 67]. Additionally, the resuspension of particulate matter that takes place during winter via sea storms [19] and water mixing [12] could foster PAHs availability and accumulation in filter feeding organisms, such as mussels, and this is particularly true for heavier and more recalcitrant PAHs [12, 68].