Effects of salinity on Macrobrachium rosenbergii physiology， biochemistry and gene expression related to molting at different molting stages

【Objective】To analyze effects of salinity on physiological and biochemical indexes，energy metabolism enzyme activity and gene expression related to molting of Macrobrachium rosenbergii，so as to provide reliable reference for M.rosenbergii cultivation in production.【Method】 300 healthy M.rosenbergii with an average initial wet body weight of 4.28±0.53 g/tail were taken as objects，and divided into 5 salinity groups of 0.2‰（control），3.0‰，6.0‰，9.0‰ and 12‰.Molting cycle，daily molting rate and molting weight gain rate of each shrimp were recorded.Pyruvate kinase（PK） activity and succinate dehydrogenase（SDH）activity of M.rosenbergii at different molting stages were determined using biochemical kits.Expression of ecdysone（MIH），retinoid X receptor（RXR），ecdysone receptor（EcR）and juvenile hormone lipase（JHEH）genes were detected by real-time quantitative PCR（qRT-PCR）.【Result】 Molting period of M.rosenberii was prolonged with increasing salinity，and the molting time of 6‰ salinity group was the longest，which was significantly higher than that of the 0.2‰ salinity group（P<0.05）.Rate of molting weight gain decreased with the increase of salinity，and the rate of molting weight gain in the salinity groups of 6.0‰，9.0‰ and 12.0‰ was significantly lower than that in the 0.2‰ salinity group.Under 5 salinity conditions，PK activity and SDH activity in hepatopancreas and gill filament reached the maximum at the late molting stage B，decreasing at the early molting stage D with the prolongation of the post-molting time.The results indicated that the energy metabolism enzyme activity of hepatopancreas and gill filaments reached the highest level in the late molting period.In the 4 molting stages，PK activity and SDH activity in hepatopancreas，gill filaments and muscle tissues increased first and then decreased with the increase of salinity.PK activity and SDH activity in muscle and hepatopancreas increased after stimulation with low salinity，while the enzyme activity decreased at 12.0‰ salinity.Relative expression of MIH gene in all salinity groups decreased significantly with the prolongation of molting.Relative expression of MIH gene in late molting stage A and late molting stage B in 0.2‰ and 3‰ salinity groups was significantly higher than that in intermolt stage C.Relative expression of RXR gene was the highest in the intermolt stage C，and was significantly lower than that in the 0.2‰ salinity group in the other molt stages（P<0.001）.Compared with intermolt stage C，relative expression of JHEH gene in all salinity groups was the highest in premolt stage D，and relative expression of EcR gene in gill filament of 3.0‰，6.0‰ and 12.0‰ salinity groups was significantly higher in premolt stage D than that in intermolt stage C（P<0.001）.【Conclusion】 Low salinity stress has great effects on molting of M.rosenbergii，leading to decreased weight gain rate and longer molting cycle，and it negatively influences production.Low salinity stress raises PK activity and SDH activity in muscle and hepatopancreas，so glycolysis rate and the tricarboxylic acid cycle in hepatopancreas and muscle are increased.However，PK activity and SDH activity are inhibited at 12.0‰ salinity group.Under salinity stress，the up-regulated expression of MIH gene and the down-regulated expression of EcR gene may lead to delay of premolt stage D and difficult molting，resulting in down-regulated expression of JHEH and RXR genes，which is the main reason for the slow growth of M.rosenbergii.The suitable water salinity for M.rosenbergii culture is 0.2‰-3.0‰.