related to direct industrial wastewater and 0.5% to domestic sewage [15]. The Mediterranean area presents a high number of natural deposits of mercury distributed along the coasts of many countries, containing about 65% of the world's cinnabar (HgS) deposits [15,16]. Particularly, in Tuscany (Italy), the levels of Hg in different environmental matrices are derived from both a natural contribution of the mineralization and the pollution caused by the huge exploitation of the area of Mount Amiata [17]. In fact, Mount Amiata, located in the south of Tuscany, is part of the geologic anomaly of the Mediterranean basin and it is characterized by a large cinnabar deposit [16,18]. Moreover, it is well known from ARPAT (acronym for Regional Agency for Environmental Protection of Tuscany) and ISPRA (acronym for Higher Institute for Environmental Protection and Research) technical reports, and scientific papers [19], the presence of mercury in the waters and organisms of the Tuscany coast and island. From 2012 to 2017, high concentrations of mercury have been reported in water, sediment and biota (as _Posidonia oceanica_, _Mytilus galloprovincialis_ and different fish species) in the Tuscan Archipelago islands, e.g., References [20, 21, 22, 23, 24].  
Many studies have been conducted worldwide on the bioaccumulation and the effects of mercury and its organic compounds in edible species, such as bivalves, cephalopods, decapods crustaceans and fish [6, 25, 26, 27]. Although, other filter-feeding organisms, such as polychatees, tunicates, sponges and barnacles, have been proposed as bioindicators in shallow waters [28], but these taxa are still little used for this purpose. Especially, sponges satisfy all the characteristics listed in References [29, 30] for a suitable bioindicator and have been recommended by many authors, e.g., References [31,32], and by the WFD as possible monitors for heavy metals. Being filter-feeders, sponges can filter a large amount of water, and they can collect and accumulate many different contaminants (such as hydrocarbons, organochlorinated compounds, heavy metals, etc.) in their tissues, which are present in the water column both in the soluble and particulate phases [10,28]. Moreover, the level of the bioaccumulation in their tissues is a function of the contaminants' concentration in the water [33]. The presence of heavy metals in sponges can affect their physiology and survival [28,34,35]. Only a few authors have investigated the presence of Hg in sponges and most of the studies have been conducted on samples, mainly on the genus _Spongia_, collected in the coast of Marseille [10,36, 37, 38].  
The current paper presents, for the first time, total mercury content in the tissue of the sponges of Montecristo and Giglio, two islands of Tuscany Archipelago National Park (TANP). The study addresses two hypotheses: (1) sponges collected show detectable total mercury contents, and (2) there are any differences in Hg concentrations between the sponge species collected in both studied islands. Our results point out that sponges have detectable Hg concentrations, showing a high inter- and intra-specific variability. This variability could also be responsible of the lack of trend in the Hg concentrations between the specimens collected from the more anthropic Giglio and the Integral Reserve of Montecristo. Anyway, the results obtained with statistical analyses give new important insights for the area on the differences among Hg concentration in sponges.