## 3 Discussion  
It is known from the literature [19; 29; 30; 31] that in marine organisms the synthesis of toxins with a protective function is usually associated with the production of unpleasant tastes and odors, interpretable in the context of chemical signaling systems in the aquatic environment. In this scenario, we have focused on the sponges _Ircinia variabilis_ (_lv_) and _Sarcotragus spinosulus_ (_Ss_), widespread and rather abundant in the Mediterranean. They both belong to the family Irciniidae, which includes species that typically produce a nauseating smell responsible for the family name, given that in Latin \"hircus\" means goat. Our investigations were aimed at identifying the pattern of molecules presumably responsible for the sponges' odor; specifically sulfur compounds, indole, and biogenic amines (BAs). For this purpose, solid phase microextraction (SPME) has been chosen as the extraction procedure, since it represents a faster and cheaper alternative to traditional methods and is solvent-free [28; 32]. Primarily, four diverse coated commercial fibers, differing in polarity and composition, were tested for the collection of volatile organic compounds (VOCs) from the headspace (HS) of a hermetically sealed vial containing the specimens of _lv_ and _Ss_ collected from the southern Italian coasts. Overall, the results of this study allowed us to assert that SPME-GC-MS was an effective technique for sampling the molecules released by sponges, including those responsible for their smell. The SPME fiber coating affected the extraction profile acquired in the chromatogram. Comparing the HS-SPME-GC-MS profiles obtained using the same type of fiber with specimens of _lv_ and _Ss_, no differences were highlighted, except with bipolar fibers, the most suitable for extracting low molecular weight sulfur compounds. Indeed, while dimethyl sulfide was produced by both species, methyl isothiamate and dimethyl trisulfide were only detectable in the profiles from _Ss_ (Table 1), suggesting a different ability to produce volatile sulfur compounds by _lv_ and _Ss_. As regards the possible role played by sulfur compounds in the sponge, their involvement in the chemical defense has been proposed for _Ircinia felix_ from the Caribbean [17]. Indeed, [PERSON] et al. submitted sulfur compounds extracted from the sponge to antimicrobial tests, showing an activity against the pathogens _Pseudomonas aeruginosa_, _Microoccus luteus_, and _Staphylococcus aureus_[17]. Conversely, a palatability test performed on teleost reef fish allowed researchers to reject the hypothesis of a deterrent effect on predators [21]. Furthermore, a possible antifouling function of sulfur compounds, already proposed in the past by [PERSON] et al. [21], cannot yet be ruled out. DI-SPME-GC-MS analysis allowed us to identify indole in sponge release solutions, by means of polyacrylate (PA) fibers, the best able to extract this compound. Another interesting difference between the species studied is the presence of indole only in the pattern of molecules secreted by _Ss_, while the levels of this chemical were undetectable in _lv_. The presence of indole could be responsible for the more intense bad odor emanating from _Ss_ compared to _lv_. Indole is originated from the amino acid tryptophan by the action of bacteria and seems to be involved in various aspects of bacterial life [33]. Other chemicals produced by the microbial metabolism having unpleasant odor are some BAs [34]. BAs are naturally formed from decarboxylation of amino acids, or their precursor. Specifically, in this study, compounds belonging to the better-known decarboxylation systems, such as histidine/histamine, lysine/cadaverine, and ornithine/putrescine, have been identified and quantified [35]. The chromatograms acquired for the two species showed very different profiles (Figure 1).