# Biomonitoring of Heavy Metals: The Unexplored Role of Marine Sessile Taxa
## 5 Conclusions and Future Perspectives  
The results of our review showed a growing interest by the scientific community in the identification of sessile taxa as nonconventional bioindicators of HMs. The increasing interest is mainly linked to the higher awareness of researchers to metals contamination in the marine environment and to the international legislations, which are more and more focused on the role of these taxa as fundamental monitors of environmental pollution. Different studies suggested that some sessile organisms can be used also in the bioremediation processes, thanks to their characteristics and the part played by the associated microbiota (bacteria and fungi). This is particularly true for sponges, cnidarians, polycheates, bivalves, and tunicates [60; 64; 89; 90; 91]. The application of vagile organisms as bioremediation and biomonitoring tools (as fish, cephalopods, or decapods crustaceans) results instead more difficult, since they are not sessile and cannot detect the actual values of a single location. In addition, even if the present work focused only on marine species, some of the considered taxa (Porifera, Cnidaria, and Bryozoa) also includes freshwater species. Therefore, these organisms can be applied as useful tools also near inland farms, industries, and plantations, as it is already done for freshwater bivalves [92]. Nonetheless, researchers should also focus their attention in identifying model organisms, which proved to be more suitable as bioindicators or bioremediation tools compared to other species of the same taxa. These research activities cannot exclude the consultations of specialized taxonomists, since (1) species belonging to the same genus can bioaccumulate different concentration of HMs [12] or can present different responses if subjected to the same metal (see Figure 1 from [PERSON] et al. [75] for the different physiological responses of _Mytilus gallprovincialis_); (2) some species considered by the literature revised for this study (e.g., _Geodia cynodium, Spongia (Spongia) lamella_, and _S. (S.) officinalis_) (see Tables S1 and S2 for citations) are included in the Annex II of the SPA/BIO Protocol of Barcelona Convention and, therefore, subjected to protection, due to their status of \"threatened\" species [93]. Our work highlighted many research gaps, but two aspects need special attention. As future perspectives, we recommend increasing sampling and collection efforts and widening the geographic and bathymetric distributions of the studies not only in the Mediterranean Sea but also in the other marine realms, provinces and ecoregions, especially in the less investigated ones, integrating the results with the studies currently available. Moreover, data standardization is still a big issue in the field, therefore, we suggest facing the problem again, reopening old programs (e.g., MED POL and PEN) or crafting new ones with new possible protocols to standardize methods, validation, analysis, and data presentation. The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/2/580/s1, Table S1: List of works divided per taxa with all document's details, Table S2: List of documents on the Mediterranean Sea province. Conceptualization, A.A. ([PERSON]) and S.P. ; methodology, A.A. ([PERSON]), C.R. and S.P. ; software, C.R., C.G., T.P.M., A.A. (Afghan Afghan); formal analysis, C.R. ; investigation, C.R. ; data curation, C.R. and A.A. ([PERSON]); writing--original draft preparation, C.R., A.A. ([PERSON]) and S.I. ; writing--review and editing, all authors; project administration, A.A. ([PERSON]) and S.P. ; funding acquisition, S.P. All authors have read and agreed to the published version of the manuscript. This study has been conducted with the financial support of PADI FOUNDATION (grant number #32694) and the Universita Politecnica delle Marche (RSA--Ricerca Scientifica di Ateneo). Not applicable.