Comparative Study Of The Effect Of Living Cells And Their Algae Powder On The Juvenile Growth Of Litopenaeus Vannamei

Litopenaeus vannamei has become one of the main economic shrimps in coastal aquaculture in our country,because of its fast growth rate,high meat yield,and rich nutrition.However,in the breeding process,the survival rate of Litopenaeus vannamei shrimp larvae often has a low range and the its breeding output is unstable.In addition to the threat of disease causing a large number of shrimp larvae to die,during the breeding process the single bait and water pollution are main reason of low-yield Litopenaeus vannamei.Therefore,in order to improve the survival rate and quality of Litopenaeus vannamei,in-depth studies on the nutritional requirements and breeding environment of Litopenaeus vannamei are required.In this experiment,the living cells of Schizochytrium limacinum,Schizochytrium limacinum powder,and Chaetoceros gracilis were fed to Litopenaeus vannamei in different combinations,to compare the effects of living cells of Schizochytrium limacinum and their algal powder on the survival rate and metamorphosis rate of larvae,and the effect of these three kinds of diets on the breeding water environment of at different concentrations.Artemia sp.were enhanced with different diets to improve its nutritional composition,the effects of enhanced Artemia sp.on the growth,digestive enzyme and immunity related enzyme activities of juvenile shrimp were further studied,so as to provide theoretical and technical support for artificial breeding of Litopenaeus vannamei.The main results of this study are as follows:（1）The living cells of Schizochytrium limacinum and their algal powder had obvious effects on the growth of the larvae of Litopenaeus vannamei.The survival rate,metamorphosis rate,digestive enzymes and immune-related enzymes of Litopenaeus vannamei were significantly affected by different diets（P<0.05）,and a single diet was not conducive to the growth and development of the larvae of Litopenaeus vannamei.Specifically,the survival rate of juvenile shrimp fed with living cells of a single species of Schizochytrium limacinum was significantly lower than that of other groups（P<0.05）.When the shrimp larvae developed from the Z2 stage to the Z3 stage,the metamorphosis rate of the Schizochytrium limacinum powder group in the single food group was significantly lower than that of the other groups（P<0.05）.The amylase,lipase activities and total antioxidant capacity of shrimp larvae fed the live Schizochytrium limacinum mixed diet were higher than those of the mixed algal powder group,indicating that the live diet group was more conducive to improving the health quality of juvenile shrimp than the algal powder group.The survival rate,metamorphosis rate,amylase activity,lipase activity,SOD activity,total antioxidant capacity and AKP activity of shrimp larvae fed by living Schizochytrium limacinum and Chaetoceros gracilis at a ratio of 1:2 were higher,while the content of MDA was significantly lower than other groups.Therefore,living Schizochytrium limacinum and Chaetoceros gracilis can be selected to feed larvae at a ratio of 1:2 to improve the survival rate and healthy quality of Litopenaeus vannamei larvae.（2）The different initial densities of the two living microalgae Schizochytrium limacinum and Chaetoceros gracilis had significant effects on the filtration rate and ingestion rate of Artemia sp..When the algal cell density was in the range of 4.68×10⁴ cells·mL^-1 to 9.77×10⁵ cells·mL^-1,the filtration rate of Artemia sp.in the high-density microalgae bait system was very low,and the ingestion rate of Artemia sp.showed a trend of increasing first and then decreasing with the increase in the density of Schizochytrium limacinum and Chaetoceros gracilis,and there was a saturation point of feed density.The survival rate of Artemia sp.decreased significantly after 12 h of microalgae nutrient intensification at different initial densities.When the density of Schizochytrium limacinum was 9.77×10⁵cells·mL^-1,the survival rate of Artemia sp.was only 37.33%.At this density of Chaetoceros gracilis,the survival rate of Artemia sp.can still reach more than 96%.The changes of fatty acid content in Artemia sp.were closely related to the nutrient components of the food they ingested.The DHA content in the Schizochytrium limacinum powder group was significantly higher than that in the living Schizochytrium limacinum group.Among them,when the single diet group（living Schizochytrium limacinum and Schizochytrium limacinum powder）,Schizochytrium limacinum powder and Chaetoceros gracilis were fed with Artemia sp.at a ratio of 2:1,compared with the control group,the EPA contents of the nauplii were increased from 0%to 0.69～0.89%,and DHA increased by 16～21 times,indicating that the appropriate concentration of Schizochytrium limacinum can effectively increase the EPA and DHA content of Artemia sp..（3）Nutritional enhancement of Artemia sp.by 1:1 between living Schizochytrium limacinum and Chaetoceros gracilis was beneficial to the growth of shrimp and the enhancement of chymotrypsin activity and lipase activity.Nutritional enhancement of Artemia sp.with the single living Schizochytrium limacinum group to cultivate Litopenaeus vannamei was beneficial to improve the AKP activity and the content of polyunsaturated fatty acids in the shrimp,and the DHA and EPA contents in the muscle of the shrimp were increased by 1.33 times and 1.27 times compared with the control group.The results indicated that Artemia sp.enhanced with suitable concentration of Schizochytrium limacinum could promote the growth and development of Litopenaeus vannamei,improve the nutritional composition of muscle.（4）High-density Schizochytrium limacinum degrades the water quality of aquaculture water seriously.Under the condition of the same mass concentration,the order of the influence of bait on the deterioration of water quality was Schizochytrium limacinum>Chaetoceros gracilis,Schizochytrium limacinum>Schizochytrium limacinum powder,it showed that when feeding living Schizochytrium limacinum cells in seedling breeding,it is necessary to balance the nutritional requirements of shrimp seedlings and water quality regulation,and adopt the method of low concentration and multiple feeding.To sum up,in the process of culturing Litopenaeus vannamei larvae,living Schizochytrium limacinum and Chaetoceros gracilis can be selected to feed larvae at a ratio of 1:2,and in the later stage of breeding,Artemia sp.were enhanced with a single species of living Schizochytrium limacinum can be selected to feed the Litopenaeus vannamei to improve the survival rate and healthy quality.