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ZHOU Qi-Cun, LIU Yong-Jian , MAI Kang-Sen  , TIAN Li-Xia  (School of Life Science, Zhongshan University, Guangzhou, 510275; Fisheries College, Ocean University of Zhanjiang, Zhanjiang, 524025) (School of Life Science, Zhongshan University, Guangzhou, 510275)  (School of Life Science, Zhongshan University, Guangzhou, 510275; Fisheries College, Ocean University of China, Qingdao, 266003)  
A single-factor experiment was conducted to study the effects of dietary vitamin C (L-ascorbyl-2-monophosphate) on growth of juvenile grouper (Epinephelus coioides), body distribution of ascorbic acid concentrations, histopathological changes and main body composition of whole body. Six iso-nitrogenous and iso-energetic diets in the levels of 0, 35, 70, 140, 280 and 700mg/kg L-ascorbyl-2-monophosphate were fed twice daily (09:00, 17:00). Triplicated groups of 20 with initial weight of about 10g were fed with the diets in the levels for 8 weeks. The experiment was done in floating net cages (1.5m×1.0m×2.0m). Results indicated that after 8 weeks of the feeding trial, weight gain (WG), specific growth ratio (SGR) and survival rate (SR) of the fish fed with the control diet (L-ascorbyl-2-monophosphate un-supplemented diet) were significantly lower than those with vitamin C enriched diets. WG, SGR and SR were the highest in the fish fed in 70mg/kg level. However, with increase of die~tary vitamin C supplemented levels from 0 to 70mg/kg, the WG and SGR increased significantly; but with increase of dietary vitamin C from 70 to 700mg/kg, the WG and SGR decreased slightly. No significant change in SR was found in the vitamin C enriched groups but the SR were higher than the un-supplemented vitamin C group. In addition, no significant differences for viserosomatic index and condition factor were shown between different groups. But it is clear that the hepatosomatic index was affected by enriched vitamin C diets, for example, the un-supplemented vitamin C group and the group in 700mg/kg level were significantly lower than other groups. Moisture, protein and ash concentration in whole body of juvenile grouper were not affected by the supplemental levels of dietary vitamin C; but the lipid concentration in the juvenile grouper increased significantly while vitamin C food enrichment from 0 to 35mg/kg, while with the increase of vitamin C supplemented level in diet was upper to 70mg/kg, the lipid concentration decreased significantly. For changes in ascorbic acid concentrations in the grouper muscle and liver, clear increases were found with the vitamin C enrichment. For instance, the ascorbic acid concentrations with no vitamin C enriched were the lowest among all, with the increase of vitamin C supplemented level in diet, the ascorbic acid concentrations increased significantly. Meanwhile, the ascorbic acid concentrations in liver were higher than in muscle's levels which were in a same supplemented level. Based on the WG and SGR of juvenile grouper, it is proposed that the optimal vitamin C enrichment in the form of L-ascorbyl-2-monophosphate for juvenile grouper growth is about 70mg/kg.
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