This study was conducted to evaluate juveniles' Ocyurus chrysurus (13.7 ± 0.45 g initial weight) to utilize lipid as an energy source on growth, feed efficiency, body composition, digestive and hepatic enzyme activities. Four diets of two protein levels (40 and 50%) with two lipid levels (6 and 12%) and 2% of digestible carbohydrates were formulated. Fish were fed for 60 days to apparent satiation at a stocking density of 10 fish per tank (100 L). Growth gain of fish fed 50% dietary protein was higher than of fish fed 40% dietary protein (P < 0.05). However, feed efficiency was significantly higher at 12 than 6% of dietary lipid. Whole-body lipid and glycogen in the liver increased dramatically with dietary lipid content-alkaline protease activity trend increased as dietary protein increased. Trypsin activity increased significantly as dietary lipid decreased, whereas chymotrypsin activity showed the opposite trend (P < 0.05). Bile salt-dependent lipase activity trend towards increasing as dietary energy decreased. Glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and fructose-1, 6-biphosphatase (FBPase) significantly increased with increasing dietary lipid levels. However, alanine aminotransferase (ALT) activity increased with dietary protein content. On the other side, pyruvate kinase (PK) activity increased with both dietary protein and lipid content. This study indicates that high dietary lipid (12%) improved the feed efficiency but did not reduce dietary protein demand in juveniles O. chrysurus.

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Controlling oxidative stress in shrimps under culture conditions is of vital importance to ensure that males have an optimal physiological status. The use of diets supplemented with vitamins are widely used for this purpose. We evaluated the effects of vitamin C on the metabolic, antioxidant, and immune responses of Penaeus brasiliensisLatreille, 1817 broodstock males under culture conditions. A formulated diet was used as a control (16 mg kg–1 of total ascorbic acid (vitamin C) content) and three diets were prepared with three vitamin C levels: 322 mg kg–1 (Diet A), 628 mg kg–1 (Diet B), and 934 mg kg–1 (Diet C). Triglycerides, cholesterol, glucose, and prophenoloxidase, total proteins, and quantity and type of hemocytes in the hemolymph were determined. We also quantified the antioxidant enzyme activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (Gpx) and lipid peroxidation in the reproductive tract. Differentially expressed genes of the immune system, namely those of α-2 macroglobulin (α2M), peneidin (Pen), hemocyanin, prophenoloxidase, and the genes encoding the antioxidant enzymes SOD, CAT, and Gpx were quantified in the tract. Sperm quantity and quality were recorded. In males fed Diet C, the lowest cholesterol values and triglycerides obtained in the hemolymph were not significantly different from those fed the control diet (P &lt; 0.05); those fed Diet C showed the highest amount of hyaline hemocytes, the lowest CAT enzyme activity, and the highest Gpx activity. The immune system genes only showed differences between α2M and Pen; sperm quantity and quality were not affected by diet. The results indicated that Diet C (13.9 g kg–1 vitamin C) was effective for the maintenance of P. brasiliensis broodstock males because it controlled cholesterol and triglyceride levels; there was less CAT activity and less activation of the immune system mediated by hemocytes.

Carnivorous common snook (Centropomus undecimalis) has potential for aquaculture, but its protein requirement in diet is 500 g kg−1 and the energetic role of lipids in this species has not been explored. Thus, this study investigated the ability of juvenile common snook (8.7 ± 1.2 g) to use dietary lipids as an energy source. The fish were fed four practical diets with two protein levels (400 and 500 g kg−1) associated with two lipid levels (60 and 120 g kg−1) for 12 weeks. Growth performance significantly improved with increasing dietary lipid levels, but no significant difference was observed at the protein level. Fish fed the 400:60 (protein: lipid level) diet exhibited significantly higher feed conversion ratios than those fed 400:120, 500:60 and 500:120 diets, but fish fed the 400:120 diet had significantly higher protein efficiency ratios. Juvenile common snook showed the ability to modulate digestive protease and lipase secretion when dietary protein or lipid levels were modified. The activity of alanine aminotransferase decreased as the dietary lipid level increased (p < .05), while fructose‐1 and 6‐bisphosphatase activities were not significantly affected by dietary protein or lipid levels. Glucose‐6‐phosphate dehydrogenase, 6‐phosphogluconate dehydrogenase and pyruvate kinase activities increased as dietary protein and lipid levels increased (p < .05). The results of this study indicate that an increase in dietary lipid level has a protein‐sparing effect and that the diet containing 400 g kg−1 protein with 120 g kg−1 lipid is suitable for growth and effective protein utilization in common snook.