# Biomonitoring of Heavy Metals: The Unexplored Role of Marine Sessile Taxa
## 1 Introduction  
Marine coastal areas are among the most exploited and vulnerable ecosystems, constantly subjected to multiple anthropogenic pressures [1, 2], due to the intensification of metropolitan areas, tourism, and industrial and agricultural activities [3, 4]. These human-induced perturbations cause in many cases the release of various contaminants, which can be transported in coastal waters through different pathways, as atmospheric depositions, rivers, industrial discharges, maritime traffic, or submarine groundwaters [3], representing important hazards for the ecosystem's health [1]. These contaminants principally include chemicals (e.g., heavy metals, polycyclic aromatic hydrocarbons (PAHs), pesticides, etc.) and fecal contaminants (e.g., _Escherichia coli_, total coliforms, and enterococci) [3]. Therefore, understanding the fate and distribution of contaminants in the sea and their origins is crucial to assess the ecological and chemical status of water and organisms and to implement management plans [2, 5]. Heavy metals (HMs) are a problematic issue for marine ecosystems, ensuring that some of these elements and their derived compounds are listed in the European Water Framework Directive (WFD 2000/60/EC) as priority pollutants. Heavy metals are naturally occurring elements that can be found throughout the earth's crust. Nevertheless, anthropogenic activities (e.g., mining and smelting operations, industrial production and useTo date, a clear definition of the term \"heavy metals\" is not yet available. The term has been widely used in chemistry but no authority such as the International Union of Pure and Applied Chemistry (IUPAC) has ever defined it [7]. Even if its use is strongly discouraged, the term is increasingly used in the scientific literature especially in articles pertaining to multidisciplinary environmental issues [8]. On a chemical basis, the group of HMs includes all the transition elements (bloc d of the periodic table), the rare earth elements (the series of lanthanides and the series of actinides, including La and Ac themselves), and all the elements presenting the typical characteristics of metals (bloc p), together with some metalloids (Ge, As, and Te) and some nonmetals (Se) [7; 9]. Some HMs (e.g., Fe, Cu, Co, Mn, and Zn) are essential elements for organisms, playing key roles in the functioning of enzyme systems. However, high concentrations induce detrimental effects in organisms and in the environment. Other metals (e.g., Al, As, Ba, Bi, Cd, Pb, Hg, Ni, Pt, and Ag) have no specific biological functions and are considered as nonessential metals [10]. In the marine environment, heavy metals can be found in various chemo-physical forms (metal ions, hydrated ions, charged metal complexes, uncharged inorganic complexes, and organometallic complexes), each of them presenting different bioavailability, toxicity, bioaccumulation, mobility, and biodegradation rates [11]. All aquatic organisms can introduce and/or accumulate HMs in their tissues, taking them up from water or food, and the concentration of these elements can vary between taxonomic levels, from phyla down to species of the same genus [12]. Many studies have been carried out on the bioaccumulation and the effects of the different elements on marine organisms, but mainly on edible species (as bivalves, cephalopods, decapod crustaceans, and fish) e.g., [13; 14]. However, other sessile organisms, e.g., polychaetes, tunicates, sponges, and barnacles, can be considered as suitable bioindicators for their physical and physiological characteristics [15; 16], and they have been recommended by many authors, e.g., [17; 18; 19; 20; 21; 22], by the WFD and by the Marine Strategy Framework Directive (MSFD) [23]. Moreover, these organisms can represent an excellent functional tool since they do not only show detectable concentrations of trace elements [18; 24] but the presence of these chemicals in their tissues can also stimulate different physiological responses [25].