Studies On Hematology And Immune Characteristics Of Sub-adult Chinese Sturgeon During Seawater Adaptation

Chinese sturgeon（Acipenser sinensis）is endemic to the Yangtze River of China.It belongs to the national first-class protected wild animal.In 2010,it was listed as a critically endangered（CR）species on the IUCN Red List.However,due to the human activities（overfishing and Gezhouba Dam,etc.）,wild populations of Chinese sturgeon have fallen drastically in the past decades,and the natural spawing activities of Chinese sturgeon has been interrupted for five consecutive years.Artificial saving has become an important means to prevent the extinction of Chinese sturgeon.However,the long-term freshwater captive environment has gradually revealed problems such as small sexually mature individuals,imbalanced ratio of male and female maturity,and decreased reproductive capacity of Chinese sturgeon.As anadromous migratory fish,Chinese sturgeon spends 90%of its natural life history in the ocean.The seawater environment plays a vital role in the growth and development of Chinese sturgeon and other physiological characteristics.Therefore,mariculture is an indispensable link in the process of marine conservation of Chinese sturgeon.In order to study the immune characteristics of sub-adult Chinese sturgeon in natural seawater,this experiment took 4-year-old Chinese sturgeon cultured in freshwater as the research sample,and cultured in seawater for 5 months after seawater domestication.Samples were collected at five time points in seawater acclimation stage（fresh water,brackish water and sea water）and mariculture stage（the second and fifth month of mariculture）.It is used to analyze the changes of blood physiological and biochemical indexes,immune tissue structure and immune gene expression during the adaptation of Chinese sturgeon juveniles to seawater.The main findings are as follows:1.Changes of blood cell parameters of Chinese sturgeon during seawater adaptationDuring seawater acclimation,HB,HCT and RBC showed a downward trend,while WBC first decreased and then increased significantly（P<0.05）;In the process of mariculture,HB,HCT and RBC were significantly increased（P<0.05）,while WBC remained stable.There was no significant change in the proportion of various types of leukocytes during the whole experiment（P>0.05）.2.Changes in plasma biochemical parameters of Chinese sturgeon during seawater adaptationDuring seawater adaptation,the levels of calcium(Ca²⁺)and phosphorus(P³⁺)were significantly increased（P<0.05）;Changes in salinity were no significant effect on MDA,AKP,ACP,and GLU.During seawater acclimation,ALB was not significant differences,SOD and TP showed an upward trend,and levels of HDLC and LZM increased significantly increased（P<0.05）;AST,TC and GLU showed a downward trend,Ig M,LDLC,TG and ALTdecreased significantly（P<0.05）;During mariculture,ALB and TP were significantly increased（P<0.05）,HDL-C was significantly decreased（P<0.05）,and the level of Ig M was significantly increased（P<0.05）.There were no significant changes in SOD,LZM,AST,ALT,TG,TC,LDL-C and GLU.3.Changes in the immune tissue structure of Chinese sturgeon during seawater adaptationObservation of immune tissues（thymus,head kidney,spleen）by tissue sectioning technique found that the thymus of juvenile 4-year-old Chinese sturgeon did not degenerate;After mariculture,the cells of head kidney tissue and spleen tissue were plump and neatly arranged,and the granulocytes in head kidney tissue and the center of melanin macrophages around blood vessels increased significantly;Lymphocytes in the spleen tissue were gathered,arranged closely,and the cell density increased significantly;However,no obvious changes were observed in the structure of the thymus.4.Expression of three immune-related genes in the immune tissue of Chinese sturgeon during seawater adaptationMHCⅡ,CD40L and LTBR were all expressed in immune tissues,and the expression levels were:MHCⅡ>CD40L>LTBR.Among them,MHCⅡhad the highest expression in spleen.During the whole experimental period,the expression of MHC Ⅱ and LTBR in the thymus decreased,and the expression of CD40L increased;The expression of MHC Ⅱ in the spleen,the expression of CD40L in the spleen and head kidney,and the expression of LTBR in the spleen had the same trend of change.They all decreased at first,then increased and then decreased.The expression of MHC Ⅱ in the head kidney first decreased and then increased.It fluctuated in a certain range,but there was no significant difference（P>0.05）;In brackish water,LTBR in head kidney was significantly increased（P<0.05）,and then recovered to a level similar to that in fresh water,and remained stable in the mariculture stage.To sum up,The seawater acclimation stage is a process of adjusting osmotic pressure and stress.During this stage,the antioxidant capacity and metabolic capacity of Chinese sturgeon have been enhanced;In the mariculture stage,due to the establishment of a new equilibrium system in the body,Ca²⁺,P³⁺,HB,HCT,and TP levels increased significantly,and most blood biochemical indicators remained stable;In the process of seawater adaptation,the physiological functions of Chinese sturgeon remained normal,and the density of cells in the head kidney and spleen tissue increased and the shape was plump.Long-term mariculture had a certain enhancement effect on the hematopoietic and immune functions of Chinese sturgeon.The research results will provide a basis for the restoration of Chinese sturgeon resources,and have important guiding significance for the reproduction and off site conservations of Chinese sturgeon.