Effect Of Three Environmental Factors On Growth And Immune Indices In Procambarus Clarkii

Procambarus clarkii is the main freshwater shrimp aquaculture in our country.Large-scale aquaculture of P.clarkii is growing rapidly.The aquaculture area and yield of rice shrimp integrated culture have been increasing continuously.P.clarkii has become the most economically valuable shrimp species in China.During the culture process,shrimp growth slows down or dies of oxidative stress due to changes in the culture environment.Juvenile P.clarkii was used as the experimental material in this study.The effects of hypoxia,low pH stress,microplastics and heavy metal stress on the growth,immunity and antioxidant properties of juvenile P.clarkii have been studied.The results are as follows:1.In the hypoxia stress（DO:1‐3 mg/L）experiment,the oxygen consumption rate and asphyxia point of juvenile P.clarkii and subadult P.clarkii increased with the increase of temperature（20-28℃）.At the same temperature,the oxygen consumption rate and asphyxia point of juvenile P.clarkii were significantly higher than that of subadult P.clarkia（P<0.05）.None of the three hypoxia stress events resulted in death within 24hours,indicating that P.clarkii has a strong hypoxia tolerance.Hypoxia significantly affects the activity and gene expression of immune antioxidant enzymes in juvenile and subadult P.clarkia（P<0.05）.The activity of SOD,CAT,LDH and LD content in the hepatopancreas of juvenile and subadult P.clarkii in the hypoxia stress group was significantly increased（P<0.05）.The expression levels of SOD m RNA,CAT m RNA,Hsp 70 m RNA and crustin 4 m RNA in hepatopancreas of juvenile P.clarkii and subadult P.clarkii in hypoxia stress group were significantly higher than those in the control group（P<0.05）,the expression levels of all genes were negatively correlated with dissolved oxygen levels.2.In the experiment of low pH（pH 3.5、pH 5.5）stress,the tolerance of juvenile P.clarkii to low pH was gradually weakened with the increase of exposure time.The LC₅₀pH value at 24 h was 2.85,the LC₅₀ pH value at 96 h was 3.44.During 21 days of low pH stress,the mortality rate was 61.24±3.01%in the pH 3.5 group,20.20±5.40%in the pH 5.5 group,and 9.62±5.73%in the control group.At the growth level:After 21days,the body weight of the pH 3.5 group increased by 48.53±8.56%,the pH 5.5 group by 92.42±13.5%,and the pH 7.5 group by 235.16±18.9%.On the level of digestive enzymes,the activities of AMS and LPS in the pH 3.5 group were significantly lower than those in the control group on day 7 and 14（P<0.05）,at 21 days,the AMS,LPS and PTS in the pH 3.5 group were significantly higher than those in the control group（P<0.05）.At the immune level,the PO,ACP and AKP in pH 3.5 and pH 5.5 treatment groups were significantly lower than those in the control group（P<0.05）,at 21 days,the LYS activity in pH 3.5 and pH 5.5 treatment groups was significantly lower than that in the control group（P<0.05）.3.In the experiment of microplastic（MPs）and heavy metal stress,microplastic particles adhered to each other and aggregated in the MPs+Cu group,and the number of microplastic particles in gill in the MPs+Cu group was significantly lower than that in the MPS group.Compared with the other two treatment groups,SOD,CAT,GPx activities and MDA content increased significantly in the MPs+Cu group and were relatively delayed.At 12 h,24 h,36 h and 48 h,the SOD m RNA expression levels in the MPs+Cu group were all significantly lower than those in the Cu group（P<0.05）.At 24 h and 48h,CAT m RNA expression in the MPs+Cu group was significantly higher than that in the Cu group（P<0.05）.Crustin 4 m RNA expressions in the MPs+Cu group were significantly higher than that in the Cu group at 12 h and 36 h（P<0.05）.The above studies indicate that:（1）hypoxia stress can induce oxidative stress response of P.clarkii,thus affecting the immune antioxidant system of juvenile and subadult P.clarkii.Compared with subadult P.clarkii,the antioxidant system of juvenile P.clarkii is more sensitive to hypoxia environment and can respond to hypoxia stress more quickly.However,the immune system of juvenile P.clarkii in hypoxia stress environment is weaker than that of subadult P.clarkii,and juvenile P.clarkii are more susceptible to pathogen infection,resulting in increased mortality.（2）Low pH stress（pH3.5,pH 5.5）will lead to the death of juvenile P.clarkii,and the death rate will increase with the decrease of pH.The stronger the damage effect on the body of juvenile P.clarkii,long-term low pH stress will lead to the suppression of digestive enzyme and immune enzyme activities,reduce the digestive and immune capacity of juvenile P.clarkii,and hinder the growth of juvenile P.clarkii.（3）The immune and antioxidant systems of juvenile P.clarkii were affected by the exposure of microplastics（MPs）and heavy metals（Cu）alone or in combination.MPs+Cu combined can cause adhesion of MPs particles to each other,making MPs particles more easily sensed by gills and reducing the accumulation of MPs particles in gills.When MPs and Cu form a complex,MPs can delay the immune toxicity caused by Cu in a short period（12 h）,but the MPs+Cu complex will produce increasingly stronger effects on P.clarkii as the exposure time increases（24h-48 h）.MPs+Cu complex can produce stronger and longer lasting toxic effects on P.clarkii.The results of this study provide scientific data for solving the environmental stress problem in rice and shrimp integrated cultivation mode,which is conducive to promoting the healthy development of P.clarkii industry.