Abstract
This study investigated the dietary effects of coated L-ascorbic acid (LA) on growth, feed utilization, survival, serum biochemical indices, immunity, antioxidant capacity, and intestinal and hepatopancreatic histology of the pre-adult red swamp crayfish. Four isoproteinous and isolipidic diets were formulated to contain several LA levels as 0, 1300, 1600, and 1900 mg/kg and designated as control (LA0), LA13, LA16, and LA19, respectively. However, the analyzed LA concentrations in diets were 0.00, 199.57, 360.45, and 487.50 mg/kg in LA0, LA13, LA16, and LA19, respectively. Triplicate treatments of crayfish (21.60 ± 0.14 g) were fed the test diets and reared in fiberglass tanks with a density of 20 individuals per each for eight weeks. Results revealed that all LA treatments had significantly enhanced growth performance compared to the control. Of interest, the LA16 treatment recorded the highest final tank biomass, biomass gain, total feed intake, condition factor, and muscle yield among the other treatments. The tank feed conversion ratio was significantly decreased in LA treatments compared to the control. Moreover, dietary LA16 and LA19 had significantly higher survival rates (93.3%) compared to (85.0%) in the LA0 group. All dietary doses of LA significantly increased serum parameters (total protein, albumin, globulin, lysozyme activity) and respiratory burst activity compared to the LA0 treatment. Dietary LA16 significantly boosted the hepatopancreatic antioxidant capacity, manifested by decreased malondialdehyde concentrations, increased catalase, superoxide dismutase, and glutathione peroxidase enzyme activities, and reduced glutathione content compared to the LA-free diet. A normal histoarchitecture of the hepatopancreatic tubules was found in all LA treatments except with some minor degenerative changes in the tubular lumen, and hepatopancreatic cells associated with enlarged nuclei were found in the LA19. However, normal intestinal histoarchitecture was found in all treatments with no recorded intestinal lesions. Of interest, the polynomial regression performed on the analyzed LA concentrations suggested that 380 mg/kg would be suitable to provide maximal biomass gain for pre-adult crayfish. In conclusion, results revealed that coated LA could enhance the growth, immunity, and antioxidant capacity of pre-adult red swamp crayfish, suggesting its potential as a functional and necessary micronutrient for crayfish diets.