The effects of dietary L-theanine on the growth performance, non-specific immunity, antioxidant status, and intestinal microflora of female Chinese mitten crabs (Eriocheir sinensis)

Abstract

L-theanine, a non-protein amino acid in tea, has been widely studied as an immune enhancer in poultry, livestock, and rodents. However, no studies have been performed on the application of L-theanine in aquaculture feed. To fill this research gap, we investigated the effects of L-theanine as a feed additive on the growth performance, immune function, antioxidant capacity, and intestinal flora of female Chinese mitten crab (Eriocheir sinensis) using biochemical assays, real-time polymerase chain reaction, and 16 S rRNA gene sequencing. In total, 300 one-year-old female Chinese mitten crabs were randomly divided into treatment groups supplemented with 0 (control), 0.5%, 1.0%, 1.5%, and 2.0% L-theanine (L0–L4, respectively; n = 4 replicates per group with 15 crabs per replicate). After eight weeks, the total edible yield was significantly higher in all treatment groups than in the L0 group (P < 0.05). Furthermore, the muscle yield (both P < 0.01) and hepatopancreas index (P < 0.01 and P < 0.05, respectively) were significantly higher in the L2 and L3 groups than in the L0 group. The gonadosomatic index was also significantly higher in the L2 group than in the L0 group (P < 0.01). Moreover, the hepatopancreas superoxide dismutase, acid phosphatase, total antioxidant capacity, and glutathione peroxidase activities increased, and the malondialdehyde content decreased in the L3 group compared to the L0 group (P < 0.05). Anti-lipopolysaccharide factor (ALF) 2 (i.e., ALF2), Toll-like receptor (i.e., TLR), and myeloid differentiation primary response gene 88 (i.e., Myd88) expression were also significantly up-regulated in the L2, L3, and L4 groups (P <0.01), and ALF1 and mammalian target of rapamycin (i.e., mTOR) expression were significantly up-regulated in the L3 group (P <0.01) compared to the L0 group. Overall, these results suggest that supplementation with 0.5–1.0% L-theanine positively affected the gut microbiota composition of female Chinese mitten crabs, improving their edible fraction, antioxidant capacity, immunity, and gut microbiota abundance.