| Phosphorus is an essential mineral for fish.The source of phosphorus in cultured fish mainly comes from dietary phosphorus supplementation.Dietary phosphorus deficiency or excess will have a negative effect on the growth,energy metabolism and bone mineralization of fish.Therefore,the phosphorus requirement is essential for fish.There are many reports on dietary phosphorus requirement in some fish species,but no study has been reported on American eel(Anguilla rostrata)in this field.The purpose of this experiment was to explore the effect of graded levels of dietary phosphorus on growth,mineral accumulation,intestinal healthy and lipid metabolism of juvenile American eel,which might provide a theoretical basis for the diet of eel.Five hundred juvenile American eels with an average body weight of 25.70 ± 0.03 g were randomly divided into five groups,each group had four replicates with 25 fish in each replicate.Five experimental diets with different phosphorus levels were prepared with sodium dihydrogen phosphate and potassium dihydrogen phosphate as phosphorus sources.The dietary phosphorus supplementation levels in basis diet were 0,0.40%,0.80%,1.20% and 1.60% respectively.The total phosphorus levels of the five diets were 0.50%,0.90%,1.30%,1.70% and 2.10% respectively.The trial period was 10 weeks.With the increasing of dietary phosphorus levels,the weight gain rate,specific growth rate,feed efficiency,feeding rate and protein efficiency rate increased firstly and then decreased,the highest values of those parameters were found in 1.30% phosphorus group.Based on the weight gain and feed efficiency,the optimum dietary phosphorus for juvenile American eel were 1.37%and 1.39%,respectively.With the increasing of dietary phosphorus level,the crude protein content of the whole fish increased firstly and then decreased,the foresaid parameters in the 1.30%phosphorus group were significantly higher than those of other groups(P<0.05).With the dietary phosphorus level from 0.50% to 1.30%,the crude lipid content of whole fish decreased significantly and the ash content increased significantly(P<0.05).However,when the dietary phosphorus level increased from 1.30% to 2.10%,the crude lipid and ash content remained stable trend.The mineral content and accumulation rate of juvenile American eel were significantly affected by the dietary phosphorus(P<0.05).With the increasing levels of dietary phosphorus,the contents of calcium,iron and zinc in whole fish increased firstly and then decreased,the highest value of those parameters were shown in the 1.30% phosphorus group;the whole fish phosphorus and copper content increased firstly and then showed a stabilized trend;the whole fish manganese content decreased significantly(P<0.05).The accumulation rate of calcium,phosphorus,copper,manganese,iron and zinc increased firstly and then decreased with the increase of dietary phosphorus level,and the maximum values of those parameters were found in the 1.30%phosphorus group.Based on the phosphorus accumulation rate,the optimum level of dietary phosphorus for juvenile American eel was 1.36%.The deformation rate in the 0.50% phosphorus group was significantly higher than those of the other groups(P<0.05),the deformation rate decreased firstly and then increased with the increase of dietary phosphorus level,and the minimum value of the deformation rate was found in the 1.30% phosphorus group.With the increasing levels of dietary phosphorus,the levels of immunoglobulin M and complement 3 levels and lysozyme activity increased firstly and then decreased,the highest values of those parameters were found in the 1.30% phosphorus group.The 1.30% and 1.70% phosphorus groups had significantly lower activities of glutamic-oxalacetic transaminase,glutamic-pyruvic transaminase and diamine oxidase activity,and levels of D-lactate and parathyroid hormone in serum than those of the other levels of dietary phosphorus groups(P<0.05),and the above parameters decreased firstly and then increased with increasing levels of dietary phosphorus,the lowest values of those parameters were found in the 1.30% phosphorus group.The level of 1,25-dihydroxyvitamin D3 increased with the increasing levels of dietary phosphorus and then maintained a stable trend,and the highest value of 1,25-dihydroxyvitamin D3 was found in the1.30% phosphorus group.With the increasing levels of dietary phosphorus,the serum alkaline phosphatase activity decreased significantly(P<0.05),and the serum phosphorus level increased significantly(P<0.05).With the increase levels of dietary phosphorus,the activities of intestinal lipase and protease of juvenile American eel increased firstly and then decreased,the values of those parameters were highest in the 1.30% phosphorus group.There was no significant difference in intestinal amylase activity among other groups(P>0.05).At the phylum level,1.30% dietary phosphorus increased the relative abundances of intestinal Firmicutes,Actinobacteria and Spirochaetota,and decreased the relative abundances of Proteobacteria and Bacteroidota.At the genus level,1.30% dietary phosphorus significantly increased the relative abundance of Cetobacterium,Faecalibacterium,Lactobacillus,and Weissella(P<0.05).With the dietary phosphorus levels increased from 0.50% to 1.30%,the levels of triglyceride and total cholesterol levels in the serum and liver as well as liver fatty acid synthase and acetyl coenzyme A carboxylase activities decreased significantly(P<0.05);the activities of lipoprotein lipase,hepatic lipase and total esterase increased significantly(P<0.05).The number of lipid droplets was higher in liver oleo-red O sections of the 0.50% and 0.90% phosphorus groups,no lipid droplets was found in the 1.30% and 1.70% phosphorus groups,and a few tiny lipid droplets were distributed in the 2.10% phosphorus group.With the increasing levels of dietary phosphorus,the expression of genes related to lipolysisnd lipid synthesis were significantly up-regulated(P<0.05),and the highest value of those parameters were found in the 1.30% phosphorus group.1.30% dietary phosphorus might strengthen the autophagy,glycerophospholipid metabolism,glycosylphosphatidylinositol metabolism,α-linolenic acid metabolism and arachidonic acid metabolism,and up-regulate the hepatic phosphatidylcholine and phosphatidylethanolamine levels were upregulated to lower the lipid accumulation.In conclusion,1.30% dietary phosphorus could improve serum non-specific immunity,increase mineral accumulation in whole fish and reduce lipid accumulation;maintain intestinal health by improving intestinal digestion and absorption capacity,improving intestinal tissue morphology and structure,and increasing the abundance of beneficial intestinal bacteria;reduce blood lipid and liver lipid levels,regulate by lowering blood lipid and liver lipid levels,regulating liver lipid metabolism enzyme activity and expression of related genes,inhibiting lipid synthesis and reducing lipid accumulation.However,the deficiency or excess of dietary phosphorus could have a negative impact on the growth,mineral accumulation and intestinal health of juvenile American eel,and thus inhibit growth.Based on the best weight gain rate,feed efficiency and phosphorus accumulation,the optimal level of dietary phosphorus juvenile American eel might be1.30%～1.39%. |
