A 126-day experiment was carried out under controlled conditions to compare the effects of mannanoligosaccharide (MOS) and fructooligosaccharide (FOS) at three levels (1.5, 3.0 and 4.5 g kg-1) in the single diets and three levels (0.75, 1.5 and 2.25 g kg-1) in the combined diets on the growth performance, nutritional efficiency indices, In vivo ADC of nutrients, digestive enzymes, hemolymph indices and finally, biological responses against 12-hr air exposure and 48-hr Aeromonas hydrophila exposure challenges of juvenile (10.56±0.32 g) crayfish Astacus leptodactylus leptodactylus. The highest values of SGR (1.59% day-1), VFI (3.80% body weight day-1), survival rate (94.1%) and the lowest FCR (2.67) were observed in the juvenile crayfish fed the diet containing 2.25 g kg-1 MOS and 1.5 g kg-1 FOS. The significantly (p<0.05) highest PER (3.27), LER (3.45), EER (3.78), PPV (71.35%), LPV (73.25%) and EPV (89.20%) were related to the diet containing 2.25 g kg-1 MOS and 1.5 g kg-1 FOS. The mean of In vivo ADCOM (73.47%), In vivo ADCCP (84.98%), In vivo ADCCF (83.79%) and In vivo ADCGE (82.77%) of the juvenile crayfish fed the diet containing MOS were higher than those of juvenile crayfish fed the diet containing FOS. The juvenile crayfish fed the diet containing 2.25 g kg-1 MOS and 1.5 g kg-1 FOS had the significantly (p<0.05) highest activities (U mg-1) for amylase (8.21), lipase (7.3) and alkaline protease (7.5) and the mean (×105 cell ml-1) of hemolymph indices (×105 cell ml-1) including THC (107.14), HC (97.18), SGC (38.12) and LGC (47.36). After 12-h air exposure challenge, the juvenile crayfish fed the combined diets showed significantly (p<0.05) higher activities of PO, SOD, LYZ and NOS than those of fed the single diets. The mean survival rate of Aeromonas hydrophila injected crayfish fed the single diets containing MOS (20.1%) was significantly (p<0.05) higher than those of fed the single diets containing FOS (15.9%) and control (11.6%). At the levels tested, 2.25 g kg-1 MOS and 1.5 g kg-1 FOS in the diet was considered optimum. It can be concluded that dietary MOS and FOS exerted positive effects on the growth performance, feed utilization and accelerated crayfish immune responses against air and bacterial exposure challenges.

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