| Rock Bream(Oplegnathus fasciatus)is an important marine species for aquaculture in the East China Sea.However,the low survival rate of its larvae significantly limits its large-scale cultivation.Nutritional regulation is an effective approach to improve the survival rate of fish larvae.Unfortunately,studies on the nutritional requirements of rock bream during the larval stage are relatively scarce,and there is a lack of specialized microdiets for larval cultivation.Therefore,it is crucial to conduct research on the nutrition of rock bream larvae to improve their survival rate.Phospholipid(PL)is an essential nutrient for growth,survival,and metabolism of marine fish larvae.However,the optimal dosage of PL in feed for rock bream larvae remains unclear.This study aimed to investigate:(1)the effects of dietary soybean lecithin(SL)levels on the growth performance,anti-hypoxia ability,and liver metabolism of rock bream larvae;(2)the optimal strategy for enhancing the effects of Artemia-enriched soybean lecithin;and(3)the differences in the growth performance,body composition,and liver metabolism of rock bream larvae fed with Artemia and microdiets during the larval stage.1.Effects of soybean lecithin on growth performance,intestine morphology,and liver tissue metabolism in rock bream(Oplegnathus fasciatus)larvaeFour types of equal-protein and equal-lipid content microdiets(MDs)were formulated with graded levels of SL to achieve phospholipid levels of(PLs,dry matter)3.84%(SL0),6.71%(SL4),9.38%(SL8),and 12.21%(SL12).Rock bream larvae(25 days post-hatching)were fed the respective MDs for 30 days with three replicates.We found that dietary SL inclusion promoted growth performance,survival rate,and stress resistance to hypoxia stress.The increased dietary SL inclusion improved intestinal structure,as shown by the increased perimeter ratio,muscular thickness,and mucosal fold height of the mid-intestinal tissue.Moreover,a high SL inclusion diet(SL12)increased the activity of the key lipolysis-related enzyme(lipase[LP])in liver tissue but decreased the activity of amino acid catabolism-related enzymes(aspartate aminotransferase[AST]and alanine aminotransferase[ALT]).RNA sequencing results in liver tissue revealed that the SL12 diet increased the transcriptional level of fatty acid activation-related genes(acs16 and acsbg2),phospholipid catabolism-related genes(acat2,lpin2,and crls),and amino acid synthesis-related genes(gs,csb,aldh18a1,and oct),but decreased the expression of amino acid catabolism-related gene gprt2.Notably,the SL12 diet significantly increased the expression of ribosome biogenesis-related genes(pes1,nop56,nop58,and rpf2)in liver tissue.The ribosome protein-related pathways were the most enriched pathways mapped in the GO database.Collectively,this study demonstrated the necessity of dietary SL for survival,growth performance,promotion of mid-intestinal morphology,and hypoxia stress during the rock bream larval stage.The SL-induced growth performance promotion was likely attributed to increasing nutrient acquisition by intestinal morphology improvement and to increasing SL catabolism and thereby sparing amino acids for protein synthesis.2.Effects of soybean lecithin(SL)enrichment SL dosage and enrichment time on nutrient composition,fatty acid composition and amino acid content of Artemia naupliiSoybean lecithin(SL,Cargill,Germany)was gently dissolved in boiling water.Newly hatched eggshells of Artemia nauplii(～2×1 05/L)were transferred into the enrichment tank.Artemia nauplii were enriched at a temperature of-28℃ with SL for 0 h,3 h,6 h,12 h,and 24 h with gradient dosages of 0 g/m3,5 g/m3,10 g/m3,and 20 g/m3 SL,respectively.Each treatment was conducted with three replicates.According to proximate nutrient composition analysis,Artemia nauplii PLs were enriched with increased SL dosage(dosage,P<0:001)and enrichment time(time,P<0:001)in an interactive mode(D × T,P<0:001).The medium SL dosage showed no significant effect on Artemia protein,moisture,and ash content(dosage,P>0:05).SL enrichment significantly increased the Artemia nauplii lipid content in a dosage-dependent manner(dosage,P<0:001).No significant differences in Artemia nauplii fatty acid composition were observed,regardless of SL dosage(dosage,P>0:05).Live prey enrichment time significantly changed the SFA,MUFA,and n-3 LC-PUFA proportions(P<0:05).Amino acid content analysis indicated that live prey enrichment time was the primary factor that affected the content of the amino acids(time,P<0:001).The enrichment protocol experiment results indicated 12 h enrichment,and 10 g SL/m3 seawater could obtain desirable results.3.A comparative study on growth performance,body composition and liver tissue metabolism rearing on soybean lecithin-enriched Artemia nauplii and micro diet in rock bream(Oplegnathus fasciatus)larvaeRock bream larvae(25 days post hatching[dph])were fed Artemia nauplii or MD for 30 days with three replicates.At Stage 1(larval 25-40 dph),significantly higher growth performance was observed in larvae fed the live prey(P<0.05).Conversely,at Stage 2(41-55 dph),feeding with MD significantly increased larval standard length,and specific growth rate compared with those of larvae fed live prey.Larvae fed a MD showed decreased lipolysis-related lipase activity as well as decreased amino acid catabolism-related alanine aminotransferase and aspartate aminotransferase enzyme activities in liver tissue.RNA sequencing revealed that feeding with the MD primarily increased the expression of lipogenesis-related genes and protein translation-related gene expression in the liver tissue.Notably,feeding with MD significantly increased ribosome biogenesis-related genes as well as mitochondria synthesis-related gene expression,indicating a high protein anabolism rate and high energy production in liver tissue.In conclusion,10 g SL/m3 seawater and 12 h could effectively enrich SL in Artemia nauplii.Retard weaning onto MD led to lower growth performance,which was likely due to the diversity of lipid and protein metabolism. |
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