Effects of different dietary selenium sources on growth performance, liver and muscle composition, antioxidant status, stress response and expression of related genes in gilthead seabream (Sparus aurata)

Selenium is an indispensable trace element for fish with different metabolic functions including protection against oxidative stress. The present study aimed to determine the effect of dietary inclusion of Se in the form of either inorganic Se (sodium selenite, NaSe) or organic Se (hydroxy-selenomethionine, OH-SeMet) on gilthead sea bream Sparus aurata. Triplicate groups of juvenile gilthead sea bream (IBW 6.16 plus minus 0.04g and IBL 7.65 plus minus 0.01cm) were distributed in 15 tanks and fed one of five practical diets for 11 weeks: basal diet with no added Se (negative control, 0.8 mg Se per kg diet), supplemented with 0.2 or 0.5 mg total Se per kg as sodium selenite (NaSe) or with 0.2 and 0.5 mg total Se per kg as hydroxy-selenomethionine (OH-SeMet) providing (1, 1.3, 1.1, and 1.4 mg total Se per kg diet respectively). After the feeding trial, fish were exposed to an acute stress by fish capture and a chronic confinement stress. Blood samples were taken at 0 and 2 h after the acute stress and after 7 days of chronic stress for cortisol analysis. The fish performance, liver and muscle composition and Se content, malondialdehyde (MDA) concentration in hepatopancreas and muscle, expression of related hepatic genes, such as superoxide dismutase (sod), catalase (cat), heat shock protein 70 (hsp70) and glutathione peroxidase (gpx), were also tested. Percentage of skeletal anomalies, histopathological alterations in liver, and lysozyme activity in serum were analysed. The highest growth rate was observed in fish fed OH-SeMet up to 0.2 mg per kg but with no significant difference with fish fed the control diet with no-added Se. The lowest growth was observed in fish fed NaSe up to 0.5 mg per kg. Increase in dietary Se, particularly in the form of OH-SeMet showed a positive correlation with Se concentration in liver and muscle. A similar relation was found in the hepatosomatic index (HSI). Thus, HSI in OH-SeMet 0.5 fed fish was significantly higher than in the control without Se supplementation, in agreement with a higher lipid content registered in these fish. Regardless of lipid peroxidation, dietary inclusion of OH-SeMet, led to a significant (P less than 0.05) reduction in MDA in both liver and muscle. However, Se inclusion in form of NaSe up to 0.2 mg per kg was not as effective as organic Se to prevent oxidative risk. Moreover, dietary inclusion of Se up to 0.2 mg per kg significantly reduced plasma cortisol after acute stress, regardless of the Se form fed. Furthermore, a slight mortality occurred during the chronic stress challenge that was stimulated by vibrio appearance and affected mostly fish fed NaSe 0.5, followed by the control group. Skeletal anomalies were not affected by dietary Se inclusion, whereas, lysozyme activity measured in serum, showed a trend of increased activity with increasing Se supplementation in diets. In summary, supplementation of Se up to 0.2 mg per kg (1-1.1 mg per kg analysed dietary Se), particularly in the form of OH-SeMet, tend to improve growth and protects juvenile gilthead sea bream from acute or chronic stress. Besides, OH-SeMet was more effective than NaSe in protection against oxidative risk in muscle.