Effects Of Dietary Lysine And Methionine Levels On Survival,Growth,feed Utilization)Gut Morphology And Immunity Of Juvenile Hybrid Grouper(Epinephelus Fuscoguttatus ♀× Epinephelus Lanceolatus ♂)

The purpose of this study was to evaluate the effects of dietary lysine and methionine levels on survival,growth,feed utilization,gut morphology and immunity of juvenile hybrid grouper(Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂),and further to use this information to determine its optimal requirements of lysine and methionine.The main experimental contents and results were as followed:1.Effects of dietary lysine levels on growth,feed utilization and related gene expression of juvenile hybrid grouper.An 8-week growth trial was undertaken to evaluate the effects of dietary lysine(Lys)levels on growth,feed utilization and related gene expression of juvenile hybrid grouper.Six isonitrogenous(53.5%of dry matter)and isolipidic(7%of dry matter)were formulated to contain 1.74%,2.18%,2.94%,3.28%,3.66%and 3.99%dietary Lys levels,A mixture containing equal proportions of aspartic acid and glycine(1:1)was used to substitute for Lys in the low-Lys diets.Triplicate groups of 15 fish(average initial body weight;11.44±0.07 g)were fed each experimental diet to apparent satiation by hand twice daily(08:00 h and 16:30 h).Results showed that WG%were significantly affected by different dietary Lys levels,and fish fed 1.74%dietary Lys had lower WG%than fish fed other dietary Lys levels.The DFI of fish fed 1.74%dietary Lys was higher than that of fish fed 2.13%,2.94%,3.28%,3.66%or 3.99%dietary Lys.The FE,PER and PPV showed similar variation trends as WG%.HSI was reduced as dietary Lys levels increased.The protein contents in whole body,muscle as well as liver of fish fed 1.74%dietary Lys were lower than those of fish fed other dietary Lys levels.The expression of hepatic IGF-1,TOR and S6K1 of fish fed 1.74%dietary Lys were significantly lower than fish fed 2.18%,2.94%,3.28%,3.66%and 3.99%dietary Lys.Based on the straight broken-line analysis of WG%against dietary Lys levels,the optimal dietary Lys requirement for hybrid grouper was estimated to be 2.16%of dry matter(corresponding to 4.05%of dietary protein on a dry weight basis).2.Effects of dietary methionine levels on survival,growth,feed utilization,gut morphology,anti-oxidation and immunity of juvenile hybrid grouper.Two separate experiments were consecutively conducted to investigate the effects of dietary methionine(Met)levels on survival,growth,feed utilization,gut morphology,anti-oxidation and immunity of juvenile hybrid grouper.In experiment 1,seven isoproteic(49.6%of dry matter)and isolipidic(7%of dry matter)experimental diets were formulated to contain 0.56%,0.89%,1.20%,1.62%,1.89%,2.22%and 2.62%dietary Met levels.Alanine was used to substitute for Met in the low-Met diets.Triplicate groups of 15 fish(average initial body weight:10.61±0.05 g)were fed each experimental diet to apparent satiation by hand twice daily(08:00h and 16:30h)for 6 weeks.In experiment 2,triplicate groups of 20 fish(average initial body weight:16.83±0.08 g)were fed the 0.56%Met contained diet and the 1.89%Met contained diet of experiment 1 for 2 weeks,and then the experimental fish were exposed to 2.0 mg Cu2+·L-1 water for 72 h after the WG%of the experimental fish was significantly different.In experiment 1,results showed that survival,growth,feed utilization,gut morphology,anti-oxidation and immunity of juvenile hybrid grouper were significantly affected by different dietary Met levels.Fish fed 0.56%dietary Met had significantly lower survival ratio than fish fed any other dietary Met levels.WG%,FE,PER and PPV were increased as dietary Met level increased,reaching a peak value at 1.89%dietary Met,and values of these indices started to reduce as dietary Met level further rose.Fish fed 0.56%dietary Met had the highest DFI among all experimental treatments.Parameters of gut morphology including hF,wF,hE,hMV in fish fed 1.89%dietary Met were better than those in fish fed other dietary Met levels except that the hMV values in foregut and midgut were slightly lower than those of fish fed 2.22%dietary Met.Fish fed 1.89%dietary Met had higher expression of pituitary GH,hepatic GHR1,IGF-1,TOR and S6K1 than fish fed other dietary Met levels.The concentration of serum LZM and IgM of fish fed 1.89%dietary Met were higher than those of fish fed other dietary Met levels.Generally,the optimal dietary Met requirement for maximal WG%,FE,PER or PPV of hybrid grouper was estimated to be 1.88%-1.91%of dry matter(corresponding to 3.79-3.85%of dietary protein on a dry weight basis).In experiment 2,the significant differences in WG%were observed between fish fed the 0.56%Met contained diet and thel.89%Met contained diet when they were fed for 2 weeks.After being exposured to 2.0 mg Cu2+·L-1 water for 72 h,fish fed 1.89%dietary Met had significantly higher survival than fish fed 0.56%dietary Met,and also,displayed activities of SOD,CAT,LZM and IgM concentration in serum,together with expression of HSP70 and Nrf2 in head-kidney were significantly higher than those in fish fed 0.56%dietary Met.