| Scatophagus argus commonly known as spotted scat,belongs to Perciformes,Scatophagidae and Scatophagus.It is mainly distributed in the south of the East China Sea,the South China Sea and Beibu Gulf region.Spotted scat is not only food fish but also ornamental fish,with high economic value.It is an important species of pond and cage culture in the coastal areas of southern China.It has a wide range of salinity adaptation and a good popularization and cultivation prospect.However,salinity changes will have a variety of effects on its growth,physiology and biochemistry,and the antioxidant system is the key line of defense against oxidative stress.In this paper,the salinity acclimation and sudden change experiments were carried out to analyze the effects of salinity on the survival,growth,and antioxidant indicators of juvenile S.argus,as well as the changing laws of gene expression and metabolic pathways when the salinity dropped sharply,aiming to provide basic data for efficient breeding of juvenile S.argus and expanding the breeding space.1.Effects of salinity on survival,growth and antioxidant defense system of S.argusIn order to explore the effects of salinity on the survival,growth and antioxidant status of juvenile S.argus,fish with average body weight of(6.27±0.28)g were exposed to different salinities groups(0,5,10,20,30,35 ppt)and reared for 60 days,and the survival rate and growth status were counted.The superoxide dismutase(SOD),catalase(CAT),glutathione peroxidase(GPx),glutathione S-transferase(GST)activities,and reduced glutathione(GSH)and malondialdehyde(MDA)contents in the liver,gills and muscle tissues of juvenile S.argus at each salinity were measured.The results showed that the survival rate of juvenile S.argus was 97.78%-100% in the range of 0～35 salinity with no significant difference(P>0.05).The body length,body weight and weight gain rate of the terminal reached the maximum when the salinity was 5,and the body length,body weight and weight gain rate of the terminal were the minimum when the salinity was 35.The SGR at salinity 5 was 0.96%±0.01%,significantly higher than that in other salinity groups(P<0.05).The quadratic regression model fitting of specific growth rate indicated that the juvenile S.argus with body weight of(6.27±0.28)g obtained the maximum specific growth rate when the cultured salinity was 7.5.The activities of SOD and CAT in the liver of juvenile S.argus in the freshwater group and the salinity 35 group were significantly higher than those in the other groups(P<0.05),and the activities of GPx and MDA in the liver were also at high levels.GST activity and GSH content had no significant difference among salinity groups(P>0.05).The study revealed that juvenile S.argus have high survival rate in the range of 0～35salinity,but grow faster in low salt environment.The fresh water and high salt environment produced a certain degree of oxidative stress in the fish,and it was recommended that the appropriate salinity range for juvenile S.argus was 5～10.2.Responses of antioxidant systems to sudden changes in salinity of juvenile S.argusIn order to study the effect of sudden change of salinity on antioxidant system of juvenile S.argus,based on the salinity of 20(control group),the juvenile spotted scats with body weight of(6.27±0.28)g were directly transferred into the water with salinity of5 and 35.Four kinds of antioxidant indexes of superoxide dismutase(SOD),catalase(CAT),glutathione(GSH)and malondialdehyde(MDA)in liver,gill and muscle of juveniles were determined at 0 h,6 h,12 h,24 h,48 h,96 h and 7 d of sudden change in salinity,and the mortality during the experiment was counted.The results showed that the survival rate of the juveniles was 100% within 7 days under low and high salt stress.With the extension of salinity stress time,the activities of SOD and CAT in liver first decreased and then increased.On the 7th day,the activities of enzyme,GSH and MDA in salinity 5group returned to the level of control group.The activities of SOD and CAT in salinity 35 group were significantly higher than those in control group(P<0.05),while the contents of GSH and MDA were not significantly different from those in control group(P>0.05).The general trend of SOD and CAT activity,GSH and MDA content in gills increased first and then decreased after salinity increasing and decreasing,and all of them remained at the level of the control group at 7 d.After the sudden increase or decrease of salinity,the activity of SOD in muscle first increased and then decreased,finally there was no significant difference compared with the control group(P>0.05).CAT activity in salinity5 group was gentle,while in salinity 35 group,CAT activity first decreased and then gradually increased to the level of control group,and finally remained stable.The content of GSH and MDA fluctuated little,and there was no significant difference between the stress group and the control group at each time point(P>0.05).The results suggested that the juvenile of S.argus had a strong adaptability to the sudden change of salinity,and the levels of enzyme and non enzyme components in the antioxidant system of various tissues increased to cope with the oxidative stress.The antioxidant index returned to the normal level after 7 d of sudden salinity change.3.Transcriptomic analysis of juvenile S.argus in response to sudden salinity dropIn order to explore the molecular mechanism of liver response to low salt stress,this study analyzed and compared the transcriptome data of liver of juvenile S.argus under low salt stress for 6 h.A total of 104 450 419 sequences were obtained by Illumina hiseq2500 sequencing.103 264 transcripts and 73 721 genes were obtained by original sequencing data quality cutting and de novo splicing.Unigenes was annotated in six databases: NR,Pfam,COG,Swiss prot,KEGG and GO.The annotation success rate was37.46%.Low salt stress produced 474 differentially expressed genes,of which 240 were significantly up-regulated and 234 were significantly down-regulated.Through functional enrichment analysis of differentially expressed gene go,it was found that low salt stress had significant effects on catalytic activity,cation binding,stimulation response,lipid metabolism process,and transferase activity.KEGG enrichment analysis showed that the pathways significantly enriched after 6 h of low salt stress were mostly related to the immune system.The expressions of hydroxylate oxidase,thioredoxin and phospholipid hydroglutathione peroxidase genes related to antioxidant function were significantly up-regulated,and the thioredoxin interacting protein genes were significantly down-regulated.The Nrf2-Keap1 signaling pathway regulating the overall expression of antioxidant enzyme lines was in the activation stage.The results showed that oxidative stress occurred in the liver of juvenile S.argus after 6 h of sudden salinity decrease,and gene expression and metabolic pathways made primary response to oxidative stress. |
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