| Litopenaeus vannamei,also known as Pacific White shrimp,with its broad salinity characteristics,the desalination farming is developing rapidly in recent years while the low efficiency in low salinity aquaculture has become increasingly prominent.In the desalination culture of shrimp,the salinity of the water is much lower than 5‰.The effect of low salinity on hepatopancreas and intestine health of L.vannamei was studied using low salinity 3‰ as a stress factor and the isotonic point of shrimp body fluid 25‰ as a control.And we selected three representative functional substances as additives to explore the effect and mechanism of nutrient regulation to alleviate shrimp.The main results are as follows:1.Effect of low salt stress on hepatopancreas and intestine health of Litopenaeus vannameiTo explore the effect of low salinity stress on hepatopancreas and intestines of Litopenaeus vannamei,shrimp were cultured at 25‰ salinity and 3‰ salinity for 8 weeks.Results showed that the weight gain rate of shrimp was significantly reduced under 3‰ salinity while the feed coefficient increased(P<0.05).And the whole shrimp ash decreased(P<0.05).The activities of hepatopancreatic digestive enzymes have an increasing trend while those of intestinal digestive enzymes are opposite.But only amylase significantly decreased(P<0.05).Low salinity stress resulted in obvious oxidative damage to hepatopancreas and intestines.Malondialdehyde content increased significantly in both tissues(P<0.05).cid phosphatase activity in the hepatopancreas increased significantly,while phenol oxidase activity decreased significantly(P < 0.05).The lysozyme activity in the intestine is significantly decreased(P<0.05).Under low salt stress,the number and the volume of B cells in the hepatopancreas of L.vannamei increased while the R cells decreased.The hepatic tubules were loosely arranged,and the basal lamina was slightly damaged;the intestinal muscle layer became thinner and the intestinal villi became shorter.Part of the microvilli of the free surface is shed.Analysis of intestinal microbial showed that Saccharibacteria,Chloroflexi and Gracilibacteria increased significantlyat phylum.The number of potential pathogens,such as Pseudomonas,Flavobacterium and Shewanella ncreased significantly,which increased the possibility of diseases outbreaks under low salinity.The pathway abundance of the following amino acid metabolism,capacity metabolism,lipid metabolism and polysaccharide synthesis,digestion and metabolism and immune system decreased significantly,while the abundance of secretion system,bacterial chemotaxis,inorganic ion transport and metabolism,protein kinase and other pathways were significantly enriched.This study showed that,although the hepatopancreatine of L.vannamei would undergo adaptive changes under the stress of low salinity 3‰,low salinity would still cause damage to the hepatopancreatine and intestinal structure,and then cause the decline of growth performance and immune-related enzyme activity,which may be closely related to the metabolic function changes of intestinal bacteria.The increase of pathogenic bacteria abundance in the gut increased the risk of disease outbreaks.2.Silymarin relieving the reaction of L.vannamei under low salinity stressUnder low salinity 3‰ stress,0(control),0.01%,0.02% and 0.04% silymarin were added respectively.The results showed that the weight gain rate of the shrimp in each dose group increased significantly,the feed coefficient decreased significantly(P<0.05).The hepatic amylase activity of 0.04% added group increased significantly,0.02% and 0.04% addition of intestinal protease and amylase increased significantly(P<0.05).Feed added 0.04% silymarin can significantly increase superoxide dismutase and catalase activity in hepatopancreas of shrimp,and increase reduced glutathione content(P<0.05).0.02% silymarin can significantly increase acid phosphatase and alkaline phosphatase activity in the intestine(P<0.05).The number of B cells and R cells increased.The basal lamina was intact,and the hepatic tubules were closely arranged.0.02% and 0.04 The % addition group can significantly increase the intestinal villi length.The addition 0.04% of silymarin significantly reduces the abundance of Gracilibacteria.At the level of genus,the abundance of Iamia,Pedomicrobium,Hyphomicrobium,Rhodococcus,Shinella,Acinetobacter,Legionella and Dietzia.was significantly higher than that of the control group,but had no significant effect on the intestinal metabolic pathway.This study showed that silymarin of 0.01~0.04% could effectively reduce tissue oxidative damage and improve growth performance of L.vannamei,but silymarin had limited effect on intestinal flora and had no significant effect on intestinal metabolic pathway abundance.3.Inulin relieving the reaction of L.vannamei under low salinity stress0(control),0.1%,0.2%,and 0.4% inulin were added to the feed.After 8 weeks of low salinity 3‰ culture,0.2% and 0.4% of the added group significantly increased the weight gain rate of the shrimp while the hepatosomatic index of the shrimp decreased significantly.And the whole shrimp ash increased significantly in the 0.4% added group.0.4% inulin can significantly increase intestinal amylase activity.0.2% and 0.4% inulin significantly increased the antioxidant levels and acid phosphatase activity of the hepatopancreas.0.2% and 0.4% inulin can effectively increase the length of the intestinal villi.The Firmicutes and Cyanobacteria of the intestines of the 0.4% inulin added group were significantly higher than the control group,and the Planctomycetes were significantly lower than the control group(P<0.05).Under low salinity stress,the addition of 0.4% inulin significantly reduced the abundance of Pseudomonas and Cellvibrio in the gut,which reduced the risk of disease outbreaks.At the same time,a variety of probiotics including Bacillus,Geobacillus,Streptococcus and Lactococcus were significantly increased after the addition of inulin.In this study,inulin(0.2%~0.4%)can effectively improve the activity o hepatopancreatine and intestinal digestive enzymes and enhance the antioxidan capacity of the body,and these effects may be closely related to the increase o probiotics abundance in the intestinal tract.4.?-glucan relieving the reaction of L.vannamei under low salinity stressAt 8-week feeding experiment was conducted to evaluate the effects of dietary ?-glucan on growth performance,hepatopancreas and intestine health of Litopenaeus vannamei at low salinity stress resistance of 3‰.Shrimp were fed with four different diets adding graded levels of dietary ?-glucan(0,0.01%,0.02%,0.04%).The results are as followes :(1)The weight gain rate increased and the feed conversion ratio decreased significantly in shrimp fed 0.02% or 0.04% ?-glucan compared to controldiet(P<0.05).Condition Factor of shrimp fed 0.04% ?-glucan was significantly higher than that in control group(P<0.05).(2)The amylase activity of the hepatopancreas in shrimp fed 0.02% ?-glucan was significantly higher than that in control group(P<0.05),while the activity of intestinal protease and amylase(in 0.02% and 0.04% ?-glucan diet)increased significantly(P<0.05).(3)The activity of SOD and GSH-PX in shrimp fed 0.02% or 0.04% ?-glucan was significantly higher than that of control group(P<0.05).(4)Furthermore,the expression of 6 genes(Toll-like receptors,IMD,HSP70,MSTN,TNF-?,CTL3)in the hepatopancreas and intestine were detected and the results show that with the increase of ?-glucan addition,the expression of Toll-like receptor and MSTN in both tissues increased,the expression of TNF-? and CTL3 decreased,and the expression of IMD and Hsp70 increased in intestinal and hepatopancreas respectively.(5)The Chao1 estimator,ACE estimator,Simpson index and Shannon diversity index of shrimp fed 0.04% ?-glucan were lower than those of control group,which indicated that the richness and diversity of intestinal microbial community were lower after adding ?-glucan.Bacillus,Chitinibacter,Geobacillus,Vibrio increased while Flavobacterium,Microbacterium,Mycobacterium decreased significantly in shrimp fed 0.04% ?-glucan compared to control group(P<0.05).In conclusion,compared with 25‰ salinity,extremly low salinity of 3‰ can cause damage to the microstructure of hepatopancreatine and intestine of L.vannamei.And meanwhile,3‰ salinity aggravate the oxidative damage of the tissue and decrease the immune ability.And 8 weeks of 3‰ salinity culture experiment showed that the addition of silymarin(0.01%~0.04%),inulin(0.2%~0.4%)and ?-glucan(0.02%~0.04%)can effectively alleviate the negative effects of very low salinity to L.vannamei and improve the growth performance of shrimp. |
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