# Biomonitoring of Heavy Metals: The Unexplored Role of Marine Sessile Taxa
## 3 Results  
### Literature Analysis at Global Scale  
A total of 919 documents (Figure 1), including duplicates, have been screened and analyzed by reading the title and abstract, using the aforementioned keywords. Those not following our eligibility criteria were removed (643), leading to 276 documents, spread among the six considered taxa (Figure 1). After excluding duplicates, the final number of analyzed works was 218 (Figure 1; Table S1). Considering all the literature found, most of the works have been published from researches with an affiliation in the United Kingdom (22), Australia (21), India (19), the United States (18), and Italy (15) (Figure 2A). While in many eastern and northern European countries and various Asian and most of the African countries, only a few studies have been published (Figure 2A). Australia and the United Kingdom were the only two, out of 44 countries, where all six taxa were studied (Table S1). Regarding the sampling area, most of the studies were carried out with organisms collected in the Temperate Northern Atlantic (32.0%) and the Western (18.7%) and Central (15.1%) Indo-Pacific marine realms (Figure 2B). Inside the Temperate Northern Atlantic realm, the Mediterranean Sea province showed the highest number of documents (34). All the other studies are scattered among the other marine realms, with only one work in the Temperate Southern Africa and a total absence of studies in the Tropical Eastern Pacific and the Arctic (Figure 2B). Up to the 12.8% of the total documents have not been taken into account for the creation of Figure 2B in as much as 12 works did not give any information about the sampling site(s), 12 considered marine organisms breed in aquarium, and 5 regarded organisms collected on hydrothermal vents located in areas not included in the division proposed by [PERSON] et al. [29] (Table S1). Most of the studies included species collected in the marine environment (92.8%), and only an 8.0% focused on estuarine organisms (Table S1). The heatmap presented as Figure 3A showed that the first document, found using the query \"heavy metal\" AND \"ascidian,\" was published before 1970 (1956, see Table S1 for citation). After this publication, for all taxa except Bryozoans, for which the first publication is registered in the decade 1991-2000 (1992, see Table S1 for citation), no works could be found until 1971-1980 (Figure 3A). In general, there is an upward trend in the number of documents through time, for almost all the taxa, while for Cirripedia, it is possible to identify a peak in 1991-2000 and a decrease in documents in the following decades (Figure 3A). The taxa of Cnidaria shows the highest number of works in almost all the decades, with the 37.0% of the publications, resulting the most studied group for HMs, followed by Porifera (18.5%), a group presenting a great increase in documents starting from 2001 (Figure 3A). From the literature collection, it was evident that most of the documents analyzed both essential and nonessential elements (72.0%), in particular Cd (146), Cu (172), Pb (110), and Zn (142), while the 19.1% analyzed only essential elements and the 8.9% only nonessential ones (Table S1). Organisms belonging to the six considered taxa have been mainly collected for biomonitoring analysis (Figure 4A), aiming to measure HMs concentrations in organisms and tissues. In particular, a significant proportion of documents in each taxon, for their analysis, processed the entire specimen, followed by the skeleton, especially in the Cnidaria group (Figure 4B). In fact, among the 43 documents on skeletal analysis, 36 examined the one of cnidarians (Table S1). Figure 4A showed also a total absence of physiological studies on Bryozoa, while only 2 have been conducted on Cirripedia (see Table S1 for citations). The remaining documents are spread among the other four taxa (Figure 4A). Works on microbiology represent a small component (15 documents) of the total, most of them (7) on Porifera (Figure 4A).