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A six-week feeding trial was carried out to evaluate the effects of different dietary Myo -inositol (MI) levels on growth, feed efficiency, immunity, and antioxidant properties of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂) fed low-fishmeal diets. Seven experimental diets were made to contain 50, 100, 169, 241, 309, 386, and 459 mg/kg of MI (analyzed values, dry matter basis), being abbreviated as MI-50, MI-100, MI-169, MI-241, MI-309, MI-386, and MI-459, and each MI treatment had three replicates. Twelve fish (average initial body weight: 18.7 ± 0.1 g) were allocated to each replicate, being hand-fed their diets to apparent satiation on 8:00 am and 16:30 pm. The results indicated that within the range of 50–241 mg/kg dietary inositol, weight gain percentage (WG%), feed efficiency (FE) and protein efficiency ratio (PER) were increased with the increasing of MI, and above the 241 mg/kg, they were dropped, especially at 386 and 459 mg/kg. According to the broken-line regression analysis conducted to assess the relationship between WG% and dietary MI levels, the optimum MI dose was 267.9 mg/kg (dry weight) in the low-fishmeal diet of hybrid grouper. Condition factor (CF), viscerosomatic index (VSI), hepatosomatic index (HSI), and intestinal index (ISI) were significantly (P < 0.05) affected by dietary MI levels, varying with a similar trend as WG%. Fish fed MI-241 had the highest values of serum lysozyme (LZM), hepatic total antioxidant capacity (T-AOC) as well as the relative mRNA level of hepatic insulin-like growth factor-Ⅰ (IGF-Ⅰ) among all treatments. In conclusion, optimum dietary MI requirement for the maximal growth of hybrid grouper was 267.9 mg/kg, and suitable dietary MI supplementations improve growth, immunity, and antioxidant properties of juvenile hybrid grouper fed low-fishmeal diets. • An optimal MI content of 267.9 mg/kg is recommended in the low-fishmeal diets of hybrid grouper. • Suitable MI supplementations improve growth, immunity and antioxidant properties of fish. • Dietary MI levels exceeded 368 mg/kg have a negative impact on fish. [ABSTRACT FROM AUTHOR]

Ascorbic acid (Vitamin C - VC) is one of the essential micronutrients required for normal growth and physiology of fish. In this research, the optimum dietary VC requirement of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) was evaluated in a comparative feeding trial and subsequent oxidative stress test. Six isoenergetic (340 kcal/kg), isoproteic (49%) and isolipidic (7%) experimental diets were supplemented with graded amounts of L-ascorbyl-2-polyphosphate (analyzed VC concentrations of 76, 86, 104, 126, 137 and 152 mg/kg dry matter) and hand-fed to fish (initial weight of 15 ± 0.1 g/fish (means ± S.D.)) in three replicate cages twice a day (08:00 h and 16:30 h) to apparent satiation for 7 weeks. Percent weight gain (WG%), together with the VC concentrations in liver and head kidney, were elevated with increasing dietary VC contents, reaching a plateau in the VC137 treatment. The optimal dietary VC requirement of hybrid grouper was estimated to be 148 mg/kg dry matter based on broken-line regression analysis of WG%. Serum immunoglobulin M (IgM) concentrations and the relative mRNA expression of hepatic insulin-like growth factor-1 (IGF-1) displayed a similar variation tendency as WG%. At the transcriptional level, the activity of Nrf2-Keap1 pathway was up-regulated by VC supplementations, with the VC104 treatment showing the highest value. In summary, the optimal dietary VC requirement for the highest value of WG% of juvenile hybrid grouper was 148 mg/kg dry matter, and appropriate dietary VC supplementation improved oxidation resistance and serum IgM concentrations of the fish. • VC affects growth and expression of hepatic IGF-1 of hybrid grouper. • At the transcriptional level, the activity of Nrf2-Keap1 pathway was up-regulated by VC supplementations. • Optimal VC requirement of hybrid grouper fed low fishmeal diets was 148 mg/kg. [ABSTRACT FROM AUTHOR]