Substitution of fish meal with krill meal in rice field eel (Monopterus albus) diets: Effects on growth, immunity, muscle textural quality, and expression of myogenic regulation factors

An 8-week feeding experiment was conducted to evaluate the effects of replacing fish meal (FM) with krill meal (KM) on the growth, immunity, muscle textural quality, and expression of myogenic regulation factors of Monopterus albus (initial body weight: 23.00 ± 0.10 g). Five experimental diets consisted of different proportions of FM (0, 25, 50, 75, and 100 g/kg) were substituted with KM, corresponding to dietary inclusion levels of 0 (KM0), 28.8 g/kg (KM29), 57.6 g/kg (KM58), 86.4 g/kg (KM86), and 115.2 g/kg (KM115). The results showed that: (1) the weight gain rate (WGR) of the fish in the KM58, KM86, and KM115 groups were significantly higher than those of the KM0 group, whereas the opposite result was observed for the feed conversion ratio. (2) Serum complement 3 content and superoxide dismutase activity in the KM58 group and alkaline phosphatase activity in the KM29, KM58, and KM115 groups were significantly higher than those in the KM0 group. However, there were no significant differences in serum immunoglobulin M, complement 4, malondialdehyde content, and catalase activity among all groups. (3) The highest levels of hardness, cohesiveness, springiness, chewiness, gumminess, and adhesiveness of muscle tissues of M. albus fed with the KM58 diet were significantly higher than those of the other treatments. (4) No remarkable difference was detected in MEF2A, MEF2D, MyoG, and Myf5 mRNA expression levels in the muscle tissues among the dietary treatments. However, MyoD2, Myh6, MyoD1, MRF4, MEF2B, and MEF2C mRNA expression levels were significantly upregulated by KM replacement. In conclusion, these results suggested that dietary inclusion levels of 57.6–115.2 g/kg KM improved the growth performance, immune function, muscle textural quality, and expression of myogenic regulation factors of M. albus. Based on the WGR, the optimal level of KM was 103.08 g/kg.