Effect of dietary copper level on the immunity, Vibrio-resistant ability, lysozyme mRNA and Toll receptor mRNA expressions in the white shrimp Litopenaeus vannamei

Experiments were conducted to determine copper accumulation, SOD and lysozyme activities, immune-related genes (Toll receptor mRNA and Lysozyme mRNA) expression level and Vibrio-resistant ability of white shrimp Litopenaeus vannamei fed the diet with different copper levels (via adding copper methionine 0, 30 and 50 mg Cu/kg) for 7d, 14d and 21d, respectively. The results indicated that the copper content in hepatopancreas of shrimp significantly increased (P0.05) with increasing dietary copper and feeding time. Copper content in muscle was always significantly lower than that in hepatopancreas (P0.05). There was no significant difference in lysozyme activities in hepatopancreas, muscle and haemolymph of the shrimp among different copper treatments (P0.05), although SOD activity of the shrimp was affected by dietary copper level as well as feeding time. The SOD activities in muscle, haemplymph and hepatopancreas of the shrimp treated with 30 mg Cu/kg diet for 21d were significantly higher than those of the shrimp treated with 50 mg Cu/ kg and 0 mg Cu/kg diets. The dietary copper level did not affect the lysozyme mRNA expression level in shrimp’s gills, but it significantly affected the Toll receptor mRNA expression level in shrimp’s gills (P0.05). The Toll receptor mRNA expression level in shrimp’s gills increased significantly (P0.05) with an increasing dietary copper for 7d (P0.05). While the shrimp treated with 30 mg Cu/kg diet showed the highest Toll receptor mRNA expression level in gills for 14d and 21d. The shrimp treated with 50 mg Cu/kg diet for 7d or with 30 mg Cu/kg diet for 14d displayed the largest half lethal time and complete lethal time when the shrimp were artificially infected with Vibrio alginolyticus. It is therefore suggested that dietary copper level affected not only the accumulation of copper in shrimp tissues, but also the SOD activity and Toll receptor mRNA expression level of the shrimp and finally affected the Vibrio resistance of the shrimp.