## 4 Discussion  
Samples show the presence of mercury both in Montecristo and Giglio. Although statistical differences in the Hg concentrations have been detected in the sponge species (_Cliona viridis_ and _Haliclona (H.) fulva_) collected in both islands of the Tuscan Archipelago, the lack of a trend excludes a possible influence of the site. In fact, even though since 1971 Montecristo has been an Integral Nature Reserve considered a pristine area not interested by mechanic impacts, such as anchoring and diving disturbances, fishing, etc., its water can still be affected by other different impacts, such as climate change and alien species, e.g., References [52; 53], and the presence of different pollutants [20; 21; 22; 23] due to water circulation [54]. Currently, no law limit is defined for Hg in sponges and few data are available on Hg concentration in this phylum, most of which regard species and genera different from those collected in this study, e.g., References [28; 55; 56]. THg values of _Sarcotragus spinosulus_, measured by us, are comparable with the concentrations found in other species belonging to the subclass Keratosa, characterized by a skeleton of spongin fibers [57] such as _Scalarisspongia scalaris_ ([PERSON], 1862) (cited as _Cacospongia scalaris_) [37], _Spongia (Spongia) lamella_ ([PERSON], 1879) (cited as _S. agaricina_) [36; 37], _S. (S.) nitens_ ([PERSON], 1862) (cited as _S. nitens_) [36] and _S. (S.) officinalis_ ([PERSON], 1759) (cited as _S. officinalis_) [10; 36; 37; 38] (Table 1). THg values of _Chondrosia reinformis_ recorded in specimens from Montecristo and Giglio are similar to the ones recorded in Reference [37] for the same species collected in different localities near Marseille (Table 1). On the other hand, results obtained for _Agelas oroides_ and _Cliona viridis_ in Reference [37] are higher than our THg values for both species (Table 1). [PERSON] et al. [37] collected samples of _Cliona viridis_ in polluted and non-polluted areas, and he did not find any differences in the Hg concentrations. Therefore, also the difference reported between our specimen and the one of Reference [37] could be related not only to the species-specificity but also to the individual specificity of sponges, with a high intra- and inter-specific variability [58; 59]. Although it is known that some heavy metals (e.g., copper, lead and vanadium) have effects on sponges, increasing their fission frequency, inducing changes in cellular aggregation and reducing growth and filtration rates [34; 35], no information is available for Hg. However, some studies showed, from laboratory experiments, that mercury can cause death, inhibition of gemulic formation and malformation in gemmoscleres in the freshwater sponge _Eplydatia fluviatilis_ ([PERSON], 1759) (0.001 to 1.000 mg\(\cdot\)kg\({}^{-1}\)) [60], and it can arrest movement of single sponge cells of _Scopalina lophyropoda_([PERSON], 1862) (1 and 5 ug-kg-1), which tended to be rounded without pseudopodia [61], while a high concentration of MeHg (0.6 mg-kg-1) induces apoptosis in tissue of _Geodia cyanonium_ ([PERSON], 1767) [62]. The highest THg values were found in _Chondrosia reniformis_ and _Sarcotragus spinosulus_, collected in Giglio and Montecristo islands, respectively. These two species share the absence of a mineral skeleton, showing _C. reniformis_, a dispersal fibrillary collagen, and _S. spinosulus_, an organic skeleton made of spongin [57]. Three papers on Antarctic and Mediterranean species [59; 63; 64] showed that spicules accumulate only a small part of heavy metals compared to the sponge tissues; while in _Spongia_ spp., the skeletal spongin fibers can trap and consequently concentrate metals, such as Fe, Pb, Cr, Zn and V [37; 65]. These studies suggest that a collagenous skeleton instead of a mineral one can bioaccumulate a higher concentration of heavy metals. On the other hand, the mercury concentrations recorded for _C.