## 1 Introduction  
Mercury (Hg) is a metal of great concern to seafood consumers and health authorities, as it accumulates in some seafood species (e.g., References [1; 2; 3]) and has a high degree of environmental persistence and toxicity [4]. The environmental presence of Hg can be attributed to both natural causes (i.e., vulcanism and geothermal sources [5]) and anthropogenic activities (e.g., coal burning, mining, cement production, oil refining, etc. [5; 6; 7]). The mechanisms of Hg speciation, transfer across biological compartments, and bioaccumulation in marine organisms depend on species-specific factors, including age, gender, life cycle (e.g., Reference [8]), and feeding and migratory behaviors [9]. Yet, previous literature evidenced that such mechanisms are also strongly influenced by the surrounding abiotic conditions (e.g., seawater temperature and pH [10; 11]). Methylmercury (MeHg; CH\({}_{3}\)Hg) is the predominant form of Hg in seafood species [2; 12; 13], as well as the most stable, bioaccumulative/biomagnifiable, and toxic form (e.g., Reference [14]), being responsible for adverse effects in humans due to its neurotoxicity, mainly neurological alterations [11; 15; 16]. From an ecological perspective, increased levels of MeHg contamination were previously associated with diminished animal resilience [17; 18], reproductive success [19], and biodiversity [20]. In line with these impacts, previous studies on marine species exposed to MeHg reported altered molecular responses, including enhanced oxidative stress, tissue damage [21], endocrine disruption, and genotoxicity [22]. Furthermore, since brain tissues are relatively permeable to MeHg [23], this compound is neurotoxic, promoting negative impacts in vertebrate species cholinergic systems, including alterations of acetylcholinesterase-cholinesterase- (AchE) activity [24; 25], as well as sensory and cognitive impairments [26]. In parallel, marine species are sensitive to changes in the surrounding abiotic conditions (e.g., temperature, pH, oxygen, and salinity), as such alterations can affect their metabolic processes [27; 28] and physiological performance, consequently compromising their resilience to stressors (e.g., chemical contamination) that can occur concomitantly. In this way, the projected climate effects, including seawater warming up by +4.8 \({}^{\circ}\)C and acidification down by \(-\)0.4 pH units [29], are expected to further push marine biota toward their thresholds of physiological tolerance, especially in species inhabiting coastal areas, which are already naturally exposed to strong tidal and/or seasonal variations, as well as to anthropogenic impacts [30]. Furthermore, the increased exposure and bio-amplification of CH\({}_{3}\)Hg in marine food webs under scenarios of climate change forcing (based on relative concentration pathways (RCP): RCP 2.6 = mitigation scenario/low emission of greenhouse gases (GHG); RCP 8.5 = high GHG emissions), triggering changes in ocean temperatures, pH (acidification), dissolved oxygen, and primary production, was demonstrated with modeling work [31]. By affecting contaminants' speciation and transfer [32; 33] and altering animal metabolism and fitness [34; 35; 36], changes to climate can negatively influence the way marine species bioaccumulate and detoxify chemical contaminants, such as MeHg, in a climate-dominant process, defined as climate change-induced susceptibility and sensitivity to pollutants [37]. Physiological alterations in marine species to overcome the stress induced by these interactive stressors can then translate into dramatic long-term implications to marine ecosystems that may compromise the sustainability of fisheries and aquaculture sectors, as well as the safety of seafood consumers [38]. _Solea senegalensis_ is a common estuarine flat fish in southwest Europe, with great economic importance to both fisheries and aquaculture sectors, due to its high market value [6]. Given its benthic behavior, _S.