Effects Of Dietary Arachidonic Acid (20:4n-6) Level On Growth Performance,fatty Acid Composition And Antioxidant Capacity Of Two Economic Juvenile Marine Crabs

In this study,the important economic crab species in China,including Scylla paramamosain and Portunus trituberculatus,were selected as the research objects.The effects of different dietary arachidonic acids(20:4n-6,ARA)on growth performance,fatty acid composition,antioxidant ability and lipid metabolism related gene expression in crab species were investigated.On this basis,the mechanism of lipid metabolism and ARA metabolism in crabs were discussed.Hope to provide theoretical support for the development and application of crab compound feed.The main research contents and results of this paper are as follows:(1)Effects of different dietary ARA levels on growth,antioxidant activity,fatty acid composition and lipid metabolism of postlarval Scylla paramamosain were studied.In this experiment,five isonitrogenous and isoenergetic diets with different dietary ARA levels(0.34%,0.48%,0.84%,1.18%and 1.81%)were used to conduct an 8-week culture experiment.There were triplicate groups of 28 postlarval Scylla paramamosain(initial weight 8.145 mg)for each diet treatment.The results showed that with the increase of dietary ARA levels,the final weight(FBW),weight gain rate(WG)and specific growth rate(SGR)were firstly increased and then decreased,and 1.18%dietary ARA level had significantly higher FBW,WG and SGR values.Survival,molting frequency(MF)and intermolt period were not significantly affected by different dietary ARA levels.Fatty acid compositions in crabs were closely related to dietary fatty acid profiles,ARA proportion in crabs significantly increased with increasing dietary ARA levels.Compared with the stable DHA content in crab,the EPA content decreased significantly with increase of dietary ARA addition(P<0.05).In crabs,superoxide dismutase(SOD)and total antioxidant capacity(T-AOC)activities generally increased with dietary ARA supplementation,and were highest in 1.18%dietary ARA group(P<0.05).The malondialdehyde(MDA)concentrations of crabs generally decreased first with the addition of dietary ARA and then increased.Crabs fed 1.81%dietary ARA showed significantly high MDA content than those fed the other dietary ARA levels.In addition,with the increase of dietary ARA content(0.34-1.18%),the relatively increased lipid content in body composition was also observed.The gene expression of fatty acid synthase(fas)in crabs increased with increasing ARA level in diets(0.341.18%)and then down-regulated expression levels with further ARA addition(1.81%).The gene expression trend of fatty acid binding protein(fabp)was similar to that offas.Under the conditions of this experiment,the effects of dietary ARA levels on the growth,lipid metabolism and antioxidant capacity of early juvenile mud crabs were comprehensively considered.It is speculated that the appropriate dietary ARA level in the compound diets of postlarval Scylla paramamosain is 1.18%.(2)Effects of dietary ARA levels on growth,antioxidant activity,fatty acid composition and lipid metabolism of juvenile Portunus trituratus were conducted.Six isonitrogenous and isoenergetic diets with different ARA levels(0.11%,0.66%,1.12%,1.63%,2.36%,2.52%)were designed.The juvenile Portunus trituratus with an initial average body weight of 0.19 g were used as the research object,and an 8-week breeding experiment was carried out.Three parallel groups of 9 swimming crabs were set up for each diet treatment.The results showed that WG,SGR,survival rate,molting frequency and intermolt period of experimental crabs were not significantly affected by dietary ARA levels.The SOD activity and catalase(CAT)activity in hepatopancreas increased firstly and then decreased with increasing dietary ARA level,and the highest SOD activity and CAT activity respectively appeared in 1.12%and 2.36%dietary ARA level(P<0.05).The MDA concentrations in hepatopancreas generally decreased firstly with increasing dietary ARA level and then increased and crabs fed 2.52%dietary ARA level showed highest MDA content.The activities of lysozyme(LZM)and CAT in the hemolymph of crabs increased firstly and then decreased with the increase of dietary ARA level,and the highest LZM activity and CAT activity respectively appeared in 2.36%and 1.63%dietary ARA level(P<0.05).Although ARA supplementation did not significantly affect the activity of acid phosphatase(ACP)in hemolymph,its activity changes were generally similar to those of CAT,and the highest values appeared in the 1.63%ARA treatment group.The gene expression of fas and relish were significantly up-regulated(0.11-1.12%)with the increase of dietary ARA level,then down-regulated with further dietary ARA supplementation.The expression of relish gene in hepatopancreas was down-regulated first and then up-regulated with the increase of dietary ARA level,and the lowest gene expression was found at 1.63%dietary ARA level.The expression of intestinal tract tlr4 was up-regulated with the ARA level in diets(0.11-1.63%)and significantly down-regulated with the further dietary ARA supplementation(P<0.05).Under the conditions of this experiment,considering the effect of dietary ARA levels on the growth performance,antioxidant capacity and related gene expression of Portunus trituratus,it is speculated that the dietary ARA level of 1.12-1.63%can satisfy the growth demand of juvenile Portunus trituratus.While excessive addition of ARA in the diet(more than 2.36%)may inhibit the growth of juvenile trituratus.In conclusion,moderate dietary ARA level can improve the growth performance of two marine crab juveniles and promote lipid accumulation,while excessive dietary ARA level may inhibit the growth of juvenile crabs as well as disrupt the original lipid balance.At the same time,moderate dietary ARA level can enhance the immune function of the two marine crabs,,while excessive dietary ARA levels may cause certain oxidative stress and increase the risk of lipid peroxidation.