Abstract
The present study aimed to determine the effects of dietary inclusion of selenium (Se) in the form of either inorganic Se (sodium selenite, NaSe) or organic Se (hydroxy-selenomethionine, OH-SeMet) on gilthead seabream (Sparus aurata). Triplicate groups of 150 fish each (6.2 +/- 0.04 g initial body weight) were distributed in 15 tanks and fed one of five experimental diets for 63 days. A control diet (C) without Se supplementation (0.8 mg Se/kg diet), was formulated and served as a basal diet for the other 4 experimental feeds, supplemented with either 0.2 or 0.5 mg Se/kg supplied in the form of NaSe or OH-SeMet. Fish growth performance, biochemical composition of liver and muscle and Se content, malondialdehyde (MDA) concentration in muscle and hepatopancreas, hepatocyte morphology and lysozyme activity in the serum were studied. Furthermore, expression of related hepatic genes, such as manganese superoxide dismutase (Mn-sod), catalase (cat), heat shock protein 70 (hsp70), and glutathione peroxidase (gpx) was also analyzed. After the trial, fish were exposed to an acute and chronic confinement stress. Blood samples for cortisol analysis were taken at 0 and 2 h after the acute stress and after 7 days of chronic stress.
The highest growth rate was observed in fish fed OH-SeMet at a level of 0.2 mg/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/kg. Increase in dietary Se, particularly in the form of OH-SeMet, led to an increase in Se contents in liver and muscle. Furthermore, dietary inclusion of OH-SeMet, led to a significant (p < .05) reduction in MDA in both liver and muscle. Se inclusion as NaSe at 0.2 mg/kg was not as effective as organic Se to improve muscle oxidative status. Dietary inclusion of Se at 0.2 mg/kg significantly reduced plasma cortisol after 2 h of acute stress, regardless of the Se form fed. Lysozyme activity measured in the serum was decreased with the increase in dietary Se supplementation.
In summary, supplementation of Se up to 0.2 mg/kg (1-1.1 mg/kg analyzed dietary Se), particularly in the form of OH-SeMet, led to a beneficial effect on growth, maintenance of hepatic morphology and improved protection of juvenile gilthead seabream from acute or chronic stress. Besides, OH-SeMet was found to be more effective than NaSe in protection against oxidative stress in fish muscle.