Haematological parameters are valuable indicators of fish health status. This study is aimed to provide baseline data of the blood profile of two teleost fish species living in different environments and with divergent feeding behaviour, namely the flathead grey mullet Mugil cephalus Linnaeus, 1758, a marine herbivorous fish, and the goldfish Carassius auratus (Linnaeus, 1758), a freshwater omnivorous fish. Using an automated system coupled with flow cytometry and light microscopy, significant variations were found between M. cephalus and C. auratus blood parameters, except for haemoglobin concentration (Hgb). A significant increase in red blood cell count (RBC) and haematocrit (Hct) levels, associated with reduced mean corpuscular volume (MCV), was revealed in mullets in respect to goldfish. These data may be attributable to differences in fish species, or to their divergent physiological activeness as high RBC values are associated with fast movement and high activity with streamlined bodies, or to environmental factors such as water salinity, an increase in which may lead to erythropoiesis as an adaptive process in seawater fish. Additionally, lower values of white blood cell count (WBC) and thrombocyte count (TC) were recorded in mullets with respect to goldfish, and these changes may be due to divergent feeding habits of the two fish species, or to their different environments since increased salinity may inversely affect WBC. Overall, findings from this study provide a better understanding of the influences of divergent environmental conditions and feeding habits on fish blood parameters. The combined use of an automatic haematological count with flow cytometry was demonstrated to be effective for an early assessment of blood parameters in different fish species.

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Clarification of the mechanisms of mercury (Hg) toxicity in fish remains challenging. An innovative approach to this clarification is the combined assessment of metabolomic shifts, alterations in the antioxidant system and the risk of oxidative damage, and Hg bioaccumulation. This strategy was applied to the livers of golden grey mullet (Liza aurata) inhabiting an Hg-contaminated system in Aveiro Lagoon, Portugal. Marked changes in both the metabolic profile and the response to antioxidants were observed in fish exposed to Hg in the environment, indicating their compromised state of health. The metabolomics analysis showed a clear differentiation between the contaminated and uncontaminated areas, revealing that Hg has multiple levels of impact, including membrane stabilization/degradation/repair processes, osmoregulation, energy metabolism, gene expression and antioxidant protection. Research on oxidative stress biomarkers showed that Hg triggered adaptive responses in the antioxidant system as seen by an increase in the activities of glutathione-S-transferase and catalase, as well as the total glutathione content, which compensated for a decrease in glutathione peroxidase activity. The integration of metabolomics and oxidative stress endpoints allowed the identification of reduced glutathione as a first line of defence against Hg and evidenced oxidative insults in cell membranes, recognized through the increase in phosphatidylcholine degradation products (phosphocholine and glycerophosphocholine). Despite these effects, the induction of lipid peroxidation was efficiently prevented. The triad approach used here was demonstrated to be sensitive and effective towards a mechanistically based assessment of Hg hepatotoxicity in fish.

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