Fish eyes and brain are highly susceptible to environmental Hg exposure but this issue is still scarcely investigated, mainly regarding methylmercury (MeHg) accumulation. Yet, Hg levels in fish lens have not been previously examined under field conditions. Total Hg (tHg), MeHg and inorganic Hg (iHg) levels were assessed in the brain, eye wall and lens of the golden grey mullet (Liza aurata) from an Hg contaminated area, both in winter and summer, together with water and sediment levels. Sampling was performed at Aveiro lagoon (Portugal) where a confined area (LAR) is severely contaminated by Hg. Fish brain, eye wall and lens accumulated higher levels of tHg, MeHg and iHg at LAR than the reference site, reflecting faithfully environmental spatial differences. The brain and eye wall responded also to the winter-summer changes found in water and sediment, accumulating higher levels of MeHg (and tHg) in winter. Contrarily, lens was unable to reflect seasonal changes, probably due to its composition and structural stability over time. The three neurosensory structures accumulated preferentially MeHg than iHg (MeHg was higher than 77% of tHg). Lens exhibited a higher retention capacity of MeHg (mean around 1 μg g(-1) at LAR), accumulating higher levels than the other two tissues. Interestingly, MeHg and iHg levels were significantly correlated for the brain and eye wall but poorly associated within the two analysed eye components. The high levels of MeHg found in the brain, eye wall and lens could compromise their functions and this needs further research.

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There are several aspects of inorganic mercury (iHg) toxicokinetics in fish that remain undeveloped despite its environmental ubiquity, bioaccumulation capacity and toxicity. Thus, this study presents new information on the uptake, distribution and accumulation of iHg following water contamination by adopting a novel set of body compartments (gills, eye wall, lens, blood, liver, brain and bile) of the white sea bream (Diplodus sargus) over 14 days of exposure. Realistic levels of iHg in water (2 μg L(-1)) were adopted in order to engender reliable conclusions in the assessment of fish health. A depuration phase of 28 days was also considered with the purpose of clarifying iHg elimination. It was found that iHg was accumulated faster in the gills (within 1 day), which also had the highest accumulated levels among all the target tissues/organs. Moreover, iHg increased gradually with exposure time in all the tissues/organs, except for the lens that showed relatively unaltered levels throughout the experiment. After 14 days of exposure, lower values of Hg were recorded in the brain/eye wall compared to the liver, which is probably related with the presence of blood-organ protection barriers, which limit iHg influx. iHg reached the brain earlier than the eye wall (3 and 7 days, respectively) and, hence, higher accumulated levels were recorded in the former. A depuration period of 28 days did not allow the total elimination of iHg in any of the tissues/organs. Despite this, iHg was substantially eliminated in the gills, blood and liver, whereas the brain and eye wall were not able to eliminate iHg within this timeframe. The brain and eye wall are more "refractory" structures with regard to iHg elimination, and this could represent a risk for wild fish populations.