## 3 Results and Discussion
### Hg Species Localization and Redistribution  
Unlike the simple image of the distribution of the mercury element, the use of two isotopically enriched tracers specific to each mercury species (\({}^{201}\)MeHg, \({}^{199}\)Hg (II)) makes it possible to produce images of the isotopic ratios 200/201 and 200/199 more accurately and specifically representing the behavior of each mercury species with time. The elemental signals were transformed into isotopic ratios according the procedure described in Section 2.5. Figure 3 shows the isotope ratio images for ratio 200/201, referring to the presence of MeHg, and ratio 200/199, referring to the presence of Hg(II), after 8 h, 7 days, and 15 days of the experiment. Isotopic ratios 200/201 and 200/199 were lower than 1.75 and 1.37 for the natural isotopic ratios, respectively, indicating an important enrichment in isotopes 201 and 199 coming from the Me\({}^{201}\)Hg and \({}^{199}\)Hg(II)) added, respectively. The results showed that Hg accumulation fingerprintings vary remarkably with the chemical form of mercury. As previously observed in zebrafish, MeHg targeted different organs than Hg(II), indicating the importance of chemical speciation on the target cells and tissues for Hg accumulation [8]. After eight hours' exposure, the isotopic ratio 200/199 did not show any enrichment in isotope 199, indicating no accumulation of Hg(II) in glass eels (Figure 2(a)). After seven days, Hg(II) was mainly observed in the gills and the olfactory bulbs, while a very low bioaccumulation was detected in the liver, the heart, and the brain (Figure 2(b)). Then, following one week in freshwater, bioaccumulation persisted in the gills, but decreased or almost disappeared in the others organs, suggesting depruation processes (Figure 2(c)). These results show that the main uptake way for Hg(II) in glass eels was through the gills. In the literature, dietary exposure has been considered as the major route of mercury uptake in fish [6], but in starving glass eels maintained in freshwater, a primary uptake through the gills is not surprising. Indeed, in contrast to marine glass eels, freshwater individuals do not have to drink to maintain their osmolarity and gills are directly exposed to water, representing a relevant interface with Hg(II). The Hg(II) way into the gills might also be made possible owing to the enhancement of the physiological gradient as a result of the counter-current principle between the flows of water and blood outside and inside the gill structures [23]. Our results also showed that a second target for Hg(II) in glass eels was an area that may correspond to the olfactory bulbs. Such an observation is consistent with previous studies conducted in larval-stage zebrafish, showing that Hg(II) can specifically target the olfactory epithelium [8; 24]. As observed for Hg(II), the main uptake way for Me\({}^{201}\)Hg after eight hours' exposure in starving glass eels was the gills (Figure 2(a)). The skin also contained Me\({}^{201}\)Hg at lower levels, which allows  
Figure 3: Isotope ratio images for ratio 200/201, referring to the presence of MeHg, and ratio 200/199, referring to the presence of Hg(II), after 8 h (a), 7 days (b), and 15 days (c) of the experiment (br: brain, ey: eye, gb: gall bladder, gi: gills, he: heart, liv: liver, ob: olfactory bulb, sp: spleen). to recognized the shape of the organism. After seven days' exposure (Figure 3b), the highest bioaccumulation of MeHg was observed in the gills and the heart, followed by the liver and the spleen. The brain showed lower values, but the lowest accumulation was observed in the muscle. Then, one week after exposure, a redistribution was observed, with the highest bioaccumulation in the muscle (Figure 3c). The heart ventricle maintained accumulation, while it decreased in the viscera and the gills. No accumulation was observed in the eye, in contrast to that observed for the endogenous \({}^{200}\)Hg (Figure 2). These results show a dynamic transport of MeHg within different organs.