Effects Of Different Stocking Densities On The Growth,Digestibility,and Antioxidant Capacity Of The Red Claw Crayfish,Cherax Quadricarinatus,Under Rice-Shrimp Symbiosis Mode

Cherax quadricarinatus is a freshwater economical shrimp with high nutritional value and market potential.In recent years,the cultivation of C.quadricarinatus is mainly based on intensive aquaculture,which can lead to the pollution problem of water ecological environment.The rice-shrimp symbiosis mode is a green agricultural production mode that combines rice and freshwater shrimp farming,which can effectively reduce the pollution problem in breeding.For Jilin Province,shrimp farming in paddy fields is in its infancy,and the farming technology is relatively backward,and there is a serious lack of basic information and related research.Stocking density is one of the most important determinants in aquaculture and directly determines the yield and size of aquatic products.At present,the optimal stocking density of shrimp farming in paddy fields in Jilin Province is still unclear,which seriously restricts the healthy and rapid development of the paddy shrimp farming industry.The research object of this study was the C.quadricarinatus,and the culture experiment lasted for 8 weeks.The healthy and complete C.quadricarinatus fry(average body weight was3.71±0.28 g)were put into the 18 experimental fields at the stocking density of 0(control),0.6(A),0.9(B),1.2(C),1.5(D)and 1.8(E)per square meter(inds/m~2).After the experiment,the indexes related to the growth,nutritional quality,digestive function,antioxidant and immune ability of red crayfish were analyzed and determined.This experiment can provide a reasonable range of stocking density for farmers when cultivating the C.quadricarinatus under the rice-shrimp symbiosis mode in Jilin Province,and provide a scientific basis for the establishment,promotion and application of the supporting technology of the rice-shrimp symbiotic culture mode.The results and conclusions of this test are as follows:1.Effects of different stocking densities on the growth and nutritional quality of the C.quadricarinatus under rice-shrimp symbiosis modeThe weight gain rate(WGR),specific growth rate(SGR),final weight,body length and body width of group B were significantly higher than those of other groups(P<0.05).The hepatopancreatic index(HSI)of group D was significantly higher than that of other groups(P<0.05).The capture rate of group A was significantly higher than those in other groups(P<0.05).The net profit in groups A,B and C was significantly higher than that in groups D and E(P<0.05).In the hepatopancreas,the gene expression levels of cathepsin L(Cat L)and retinoid X receptor(RXR)in group A were significantly higher than those in other groups(P<0.05).In the the muscle,the expressions of Cat L and RXR in group B were significantly higher than those in other groups(P<0.05).The crude fat content in groups A and B were significantly higher than that of other groups(P<0.05).Crude protein content in group B was significantly higher than those in other groups(P<0.05).There were no significant differences in the contents of total essential amino acids(TEAA),semi-essential amino acids(SEAA),delicious amino acids(DAA)and non-essential amino acids(NEAA)among the density groups(P>0.05),but they all had the highest values in group B.The essential amino acid score(EAAS)of groups A and B was higher than 100,and the mean value of group B was the largest.The content of SFA in group B was the lowest.the content of PUFA in group C was the highest.The MUFA content of group A was significantly higher than that of other groups(P<0.05).The content of high unsaturated fatty acids(HUFA)in group C was the highest,which was significantly higher than that in other groups(P<0.05).The n-3 PUFA content of group B was significantly higher than that of other groups(P<0.05).The ratios of docosahexaenoic acid(DHA)/eicosapentaenoic acid(EPA)and n-3/n-6 in group B were significantly higher than those in other groups(P<0.05).2.Effects of different stocking densities on the digestive capacity and the intestinal microbial flora of the C.quadricarinatus under rice-shrimp symbiosis modeIn hepatopancreas,the activities of lipase,amylase and pepsin in group B were significantly higher than those in other density groups(P<0.05).In intestinal,amylase activity in group A was significantly higher than that in other groups(P<0.05),lipase and pepsin activities in group B were significantly higher than those in other groups(P<0.05).The ultramicroscopic observation of the hepatopancreas of the C.quadricarinatus in each density group showed that the hepatocytes in groups D and E were severely damaged,the number of lipid droplets was relatively small,with obvious vacuoles.In groups A,B,and C,the degree of damage to the hepatocytes was relatively small,the number of lipid droplets was abundant.The ultramicroscopic observation of the intestines of the C.quadricarinatus in each density group showed that the intestinal epithelial cells of the five density groups were all edema to varying degrees.Epithelial cells in group A were slightly edematous.The intestinal epithelial cells in groups B,C,D,and E showed moderate edema.At the phylum level,the dominant phyla were Tenericutes,Proteobacteria,Firmicutes,Fusobacteria and Bacteroidetes.At the genus level,the major intestinal flora were Bacillus(Candidatus Bacilloplasma and Candidatus Hepatoplasma)and Aeromonas(Aeromonas,Hypnocyclicus,Vibrio).It can be seen from the Venn diagram that the number of common OTUs in the intestinal flora of the Cherax quadricarinatus in 5 density groups were 52,each group had independent OTU species.3.Effects of different stocking densities on the antioxidant and immune capacity of the C.quadricarinatus under rice-shrimp symbiosis modeIn hepatopancreas and muscle,the activities of superoxide dismutase(SOD),total antioxidant capacity(T-AOC),catalase(CAT)and glutathione peroxidase(GSH-Px)in group B were significantly higher than those in other groups(P<0.05),and the content of MDA was significantly lower than other groups(P<0.05).In the hepatopancreas and muscle,the gene expression levels of copper zinc superoxide dismutase(Cu Zn-SOD)and glutathione transferase(GST)in group B were significantly higher than those in other groups(P<0.05).There was no significant difference in the expression level of GPx gene in the hepatopancreas between groups A and B,but was significantly higher than that in other groups(P<0.05).The expression level of GPx gene in muscle of group A was significantly higher than that of other groups(P<0.05).In the hepatopancreas,the activities of alkaline phosphatase(AKP)and acid phosphatase(ACP)in group B were significantly higher than those in other groups(P<0.05).In muscle,the activity of AKP in group B was significantly higher than that in other groups(P<0.05),and the activity of ACP in group A was significantly higher than that in other groups(P<0.05).In the hepatopancreas and intestine,the gene expression levels of Down syndrome cell adhesion factor(Dscam)and antimicrobial peptide(Crustin)in group B were significantly higher than those in other groups(P<0.05).The gene expression level of heat shock protein 70(HSP70)in the hepatopancreas of group B was significantly higher than that of other groups(P<0.05).The expression level of HSP70 gene in the intestine of group A was significantly higher than that of other groups(P<0.05).In conclusion,under the conditions of this experiment,appropriate stocking density(0.9inds/m~2)can improve the growth performance,nutritional quality,digestive function,antioxidant and immune capacity of the C.quadricarinatus in paddy field,the diversity of microflora in the gut is high,and the economic benefits are maximized.In view of the above aspects,it is recommended that the stocking density of the C.quadricarinatus cultured under the rice-shrimp symbosis mode in Jilin Province is 0.8-1.0 inds/m~2.