| Resumo: | Every animal uses the eyes to monitor the surrounding environment and maintain homeostasis. Mercury (Hg) (including methylmercury - MeHg) is a pernicious and ubiquitous contaminant in natural waters but the assessment of its potential to interfere with fish eyes is an almost unexplored issue. This thesis will contribute to fill this knowledge gap by the evaluation of mercury accumulation in grey mullet eye wall (Liza aurata) together with the assessment of biochemical endpoints related with the oxidative stress status. This approach was complemented by the characterization of environmental contamination profiles (both in water and sediment). The study was performed at Aveiro lagoon (Portugal), where a well-established mercury historical contamination gradient provides a good background for the assessment of mercury accumulation and its toxic effects under realistic conditions. Sampling was conducted in two sites distancing around 10 km, namely: Largo do Laranjo (LAR) located in the most contaminated area, and São Jacinto (SJ) closer to the lagoon inlet and selected as reference site. Keeping in view that water temperature could affect Hg accumulated levels and biochemical responses, sampling was carried out both in winter (February 2013) and summer (June 2013). L.aurata specimens (n=20) were caught at each site/season and eye wall was removed and preserved in the field. Eye wall was analysed for total Hg and MeHg levels, as well as for antioxidant responses (catalase- CAT, SOD – superoxide dismutase, glutathione peroxidase- GPx, glutathione reductase- GR, glutathione–S-transferase - GST) and peroxidative damage (LPO). Inorganic mercury levels were estimated from total mercury and methylmercury levels. Total mercury, inorganic mercury and methylmercury in eye wall were higher at LAR than SJ in winter and summer, reflecting environmental spatial differences of water column and surface sediments. Moreover, fish caught at LAR in winter showed a significant decrease of CAT and SOD, while GST and GR exhibited the same tendency but without statistical support. Peroxidative damage was also recorded as indicated by the LPO enhancement. A much more homogenous spatial pattern was recorded for those biological endpoints in summer, since only the increment of GR and GPx was noticed at LAR. Cause-effect relationships between accumulated mercury (inorganic and methylmercury) were searched by the principal component analysis (PCA). An association between both forms of mercury and GPx and LPO was discerned by PCA. Current data point out the vulnerability of fish eyes to water contaminants, namely mercury. This neurotoxicant can be accumulated in eye wall leading to alterations in the cellular protection against oxidative stress. Such repercussions could eventually compromise fish performance and survival.
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