Lysine is known to be a functional nutrient from yeast to human beings. However, the mechanisms for lysine to regulate body growth and metabolism in fish species are far from fully illuminated. In order to elucidate the molecular effects of lysine on growth performance and metabolism, largemouth bass (Micropterus salmoides), which is extremely sensitive to protein composition, was chosen as the experimental model. Juvenile largemouth bass (4.16 ± 0.02g) were fed with the lysine control diet (LC diet), the lysine supplementation diet (LS diet), and the low lysine diet (LL diet) for 8 weeks. The results showed that the LS diet group significantly increased fish growth, feed efficiency, and nutrient retention compared with those of the LL diet group in largemouth bass. Moreover, compared to the LL diet, lysine supplementation also elevated the plasma total protein, total EAA, total AA, and some individual AA concentrations. Mechanistically, lysine supplementation had a significant effect to decrease the mRNA expression levels of peptide and AA transporters induced by lysine restriction. Lysine supplementation also had an important impact on regulating the target of rapamycin (TOR) signaling pathway. More importantly, the key regulators in the amino acid response (AAR) signaling pathway were also down-regulated by lysine supplementation. Our results provide a clear elucidation of how dietary lysine affected growth performance, physiological and biochemical responses, and signaling responses and represent a sound foundation for using lysine to improve the nutrient utilization of poultry by-product meal in largemouth bass.

Xanthophyll has multiple physiological functions to improve the quality of farmed animals. The present study aimed to investigate the effects of xanthophyll on the growth performance, antioxidation, immunity, pigmentation and meat quality of southern catfish (Silurus soldatovi meridionalis Chen). Juvenile southern catfish (18.35 ± 0.04 g) were randomly allocated into 24 cages (30 juveniles per cage), and fed diets with different dietary xanthophyll levels (at 14, 42, 80, 108, 126 and 152 mg/kg, dry matter of diet) twice daily for 8 weeks. Results indicated that the diet with 80 mg/kg xanthophyll induced a higher specific growth rate (SGR), feed efficiency ratio and protein productive value. Moreover, the 80 mg/kg xanthophyll diet also increased complement 3, immunoglobulin M (IgM) and lysozyme content more than the other groups. The mRNA expression level of inflammation-related genes and antioxidant enzyme activities further confirmed the effects of 80 mg/kg dietary xanthophyll on improving immune response. The present study also found that the 126 mg/kg xanthophyll diet significantly enhanced the content of total carotenoids and xanthophyll, hydroxyproline, collagen and amino acid in muscle. The diet with 126 mg/kg xanthophylls also induced lower drip loss, thawing loss, centrifugal loss, cooking loss and higher muscle adhesiveness, cohesiveness, springiness, gumminess and chewiness than the other treatments. In conclusion, quadratic regression model analysis based on SGR and IgM revealed that the optimum xanthophyll level in the diet was 86.78 and 84.63 mg/kg, respectively. Moreover, broken line regression analysis based on xanthophyll content in dorsal skin and chewiness in muscle demonstrated that the optimal xanthophyll level was between 89.73 and 108.66 mg/kg in the diet of southern catfish (S. soldatovi meridionalis Chen).

Our previous study demonstrated that based on growth performance and feed utilization, cottonseed meal (CSM) could substitute 20% fishmeal (FM) without adverse effect on golden pompano (Trachinotus ovatus). Muscle deposition was also an important indicator to evaluate the efficiency of alternative protein sources. Therefore, the present study was conducted to explore the changes of physiobiochemical and nutrient metabolism in muscle after FM replaced by CSM. Four isonitrogenous and isolipidic experimental diets (42.5% crude protein, 14.0% crude lipid) were formulated to replace 0% (CSM0 diet), 20% (CSM20 diet), 40% (CSM40 diet), and 60% (CSM60 diet) of FM with CSM. Juvenile fish (24.8 ± 0.02 g) were fed each diet for 6 weeks. The results presented, which, compared with the CSM0 diet, CSM20 and CSM40 diets, had no effect on changing the muscle proximate composition and free essential amino acid (EAA) concentration. For glycolipid metabolism, the CSM20 diet did not change the mRNA expression of hexokinase (hk), glucose transport protein 4 (glut4), glucagon-like peptide 1 receptor (glp-1r), while over 20% replacement impaired glucose metabolism. However, CSM20 and CSM40 diets had no effect on altering lipid metabolism. Mechanistically, compared with the CSM0 diet, the CSM20 diet did not change muscle nutritive metabolism through keeping the activities of the nutrient sensing signaling pathways stable. Higher replacement would break this balance and lead to muscle nutritive metabolism disorders. Based on the results, CSM could substitute 20–40% FM without affecting the muscle nutritive deposition. All data supplemented the powerful support for our previous conclusion that CSM could successfully replace 20% FM based on growth performance.

The present study was designed to elucidate the changing pattern of growth performance, nutrient metabolism, antioxidant defense, and immune response after fishmeal (FM) was replaced by cottonseed meal (CSM). Four isonitrogenous and isolipidic experimental diets (42.5% crude protein, 14.0% crude lipid) were formulated to replace 0% (CSM0 diet), 20% (CSM20 diet), 40% (CSM40 diet), and 60% (CSM60 diet) of the FM protein with CSM. Juvenile golden pompano (Trachinotus ovatus) with an initial body weight of 24.8 ± 0.02 g were fed each diet for 6 weeks. The results showed that compared with the control diet, significant differences were not found in the weight gain ratio, specific growth performance, and apparent digestibility of dry matter and protein after 20% FM was replaced by CSM. Higher CSM replacement markedly decreased the growth performance (P < 0.05). Moreover, the CSM20 and CSM40 diets did not depress the feed efficiency ratio and protein efficiency ratio compared with the CSM0 diet. Further exploring the physio-biochemical and molecular responses, the present study also showed that dietary 20–40% CSM replacement had no significant effect on altering the whole body, plasma nutrient contents, free amino acids, or whole-body fatty acid contents. Additionally, the CSM20 diet did not change the mRNA and protein phosphorylation levels of the key enzymes and regulators involved in target of rapamycin (TOR) and amino acid response (AAR) signaling, nutrient metabolism, immune response, and antioxidant defense. Higher CSM inclusion significantly varied the TOR and AAR signaling response, nutrient metabolism, immune response, and antioxidant defense. Based on the results, CSM could successfully replace 20% FM in the formulated diets for golden pompano. This study proposes the optimal inclusion level of CSM in the diet, which has an important significance in optimizing aquafeeds formulation.