# Biomonitoring of Heavy Metals: The Unexplored Role of Marine Sessile Taxa
## 4 Discussion  
At present, the sessile taxa considered in this study (Porifera, Cnidaria, Bryozoa, Polychaeta, Cirripedia, and Tunicata) are still little used as biomonitors compared with other groups, despite the fact that their potential application has been highlighted repeatedly during time [12; 23], with an increasing annual trend in the last years. For example, searching on Scopus database using the keywords \"heavy metal\" AND \"bivalve\" or \"heavy metal\" AND \"fish,\" without conducting the screening, 1148 and 6800 documents were obtained, respectively, more than all the works screened in our research, conducted using many keywords. In fact, the vast majority of the studies on the topic referred to organisms representing food resources for human consumption, which do not have to exceed specific levels, established by Community legislation or other relevant standards [23; 30]. Another factor to take into account is that the considered taxa are not of the same economic importance as bivalves or fish, even though the economic role covered by some of them (e.g., sponges) is more and more often highlighted by the scientific community [31]. Nonetheless, looking at the physical and physiological characteristics a bioindicator should present (see [PERSON] et al. [15] and [PERSON] et al. [16]), these groups fit in many of the listed features. In fact: (i) with the exception of some cnidarians and polychaetes, they are exclusively sessile; (ii) most of them are active filter-feeders; (iii) they present a global distribution, therefore they can be collected and used for this purpose all around the globe, even at the Poles; (iv) these taxa comprehend modular organisms [32], therefore the collection of small portions does not affect their survival and they can rapidly regenerate; (v) some of them present also a gregarious behavior [33; 34; 35; 36; 37], allowing to sample just a part of a population and to observe possible changes on long-term monitoring. From our review, it is clear that, at a global scale, studies are not equally distributed among marine regions [29], with most studies focused the Mediterranean Sea province [29], probably due to its nature of being a semienclosed basin [38], and thus more subjected to anthropogenic stress and pollution [39], as the one caused by HMs (see Table 1 from [PERSON] [40], for quantities released annually, and Table 1 from [PERSON] et al. [41], for the concentration in the Mediterranean water). The basin, moreover, is subjected to the European legislation (e.g., MSFD 2008/56/EC), which suggests biological indicators (e.g., sponges and cnidarians) as potential tools to monitor the environmental status of coastal areas. Therefore, researchers could be more addressed to use unconventional organisms in their biomonitoring studies. However, also in the Mediterranean basin, there is a bias in the studies distribution, since research activities mainly focused on the Western Mediterranean ecoregion (22) [29], while the others are interested only by a low number of studies. This condition should be explained with the affiliation of the researchers studying the topic, which are mostly present in countries facing the Western Mediterranean ecoregion. Moreover, most of studies are limited to the most accessible shallow waters, mainly investigated by scuba diving. Another strong bias highlighted by the present work is the difference in the number of documents per taxon, most of them concerning Cnidaria (37%) and Porifera (18.5%). Considered cnidarians are especially from tropical areas (Table S1), in which studies principally investigated the concentrations of HMs particularly in the hard skeleton of many scleratinian corals. Their skeleton is, in fact, characterized by the presence of growth bands, used in the determination of coral's age [42; 43], and as a proxy of ocean responses to environmental factors, as temperature and salinity [44; 45], and also contaminants, such as heavy metals [46; 47], hydrocarbons [48], and pesticides [49].