# Biomonitoring of Heavy Metals: The Unexplored Role of Marine Sessile Taxa
## 3 Results
### Literature Analysis Focused on the Mediterranean Sea Province  
The literature analysis on documents found within the Elsevier's Scopus database led to the record of 34 works on organisms collected in the Mediterranean Sea province. The manual research carried out on the references reported in these works led to the addition of seven documents. The total 41 documents (Figure 1) regarded 5 of the 6 considered taxa: Porifera (24), Cnidaria (3), Polychaeta (4), Cirripedia (2), and Tunicata (8) (Table S2).  
Among all documents, only one work regarding Porifera was published before 1991 (1990, see reference in Table S1) (Figure 3B). The temporal distribution of documents showed how Porifera were considered in every 5-year period, with a peak in 2006-2010 (Figure 3B). Polychaeta were included in a constant number of works from 2001 to 2005, while for Tunicata, there is an increment of documents during time (Figure 3B). On the other hand, the low number of works on Cnidaria and Cirripedia is scattered through the different 5-year periods, from 2001 to 2005 till today (Figure 3B).  
Inside the basin, most of the studies collected organisms in the Western Mediterranean ecoregion (22), mainly along the French and Spanish coasts (Figure 6; Table S2), followed by the Adriatic Sea (7), the Ionian Sea (6), the Levantine Sea (4), and the Aegean Sea (2) (Table S2). Figure 6 shows that most of the Porifera have been collected in the Western Mediterranean, while almost all studies on Tunicata and all on Cirripedia are referred to organisms collected in the Levantine and the Aegean Sea. Two-third of documents, moreover, gave information on the sampling depth (Figure 7). Most of the organisms were collected between 0 and 5 m depth, while Porifera is the only taxon collected at all the considered depth ranges (Figure 7).  
Figure 6: Map of the distribution of documents per taxa in each ecoregion of the Mediterranean Sea.  
Figure 7: Donut chart of the percentage of documents giving information of the sampled depth(s) (**left**), and number of documents per taxa per depth range (**right**).  
Most of the documents studied the role of organisms as bioindicators (28) especially of Cu (28), Pb (23), and Cd (20) contamination. Fourteen works explored the main effects of Cd (10) and Cu (9) on the collected species (Table S2), while one paper highlighted the tolerance of the microbiota extracted from polychaetes to Cd, Cu, and Zn (see Table S1 for citation).  
Since Porifera resulted to be the most studied taxa, we considered this group for further considerations. Species studied belong to 11 orders of sponges (Table S2), and Dictyacettida is undoubtedly the one to which most of works on biomonitoring are related. These organisms showed Cd concentrations ranging between 0.2 and 1.60 mg-kg-1 d.w., Cu between 22 and 300 mg-kg-1 d.w., and Pb between 0.1 and 90 mg-kg-1 d.w. (Table S2). Dictyacettida has not been investigated for possible effects given by the presence of trace elements, but orders as Poecilosclerida and Scopalinida were considered more suitable for physiological studies and mainly on Cd and Cu (Table S2).