A 56-day feeding trial was conducted to evaluate the effect of different dietary protein sources on the growth performance, muscle flavor substances, and quality structure of Triploid Crucian Carp. Three isonitrogenous (32.00%), isolipidic (8.00%), and isoenergetic (18.00 MJ kg⁻¹) practical diets were formulated. These diets consisted of fishmeal as the animal-derived protein source (AP), a combination of soybean meal and rapeseed meal as the plant-derived protein source (PP), and a mixture of fishmeal, soybean meal, and rapeseed meal as the mixed protein source (MP). Each diet was randomly assigned to triplicate tanks of fish and each tank was stocked with 25 fish (11.5 ± 0.4 g). The fish were fed until apparent satiation twice a day. The results showed a significant enhancement in the growth performance of Triploid Crucian Carp in the AP group compared with both the MP and PP groups (p < 0.05). Dietary plant derived protein can remarkably reduce the crude lipid content and increase the moisture content of the whole body and the dorsal muscle (p < 0.05). The antioxidant ability of fish in the PP group and MP group was better than that in the AP group (p < 0.05). Regarding free amino acids composition of muscle, the contents of glycine, methionine, and lysine were significantly enhanced in the AP group, while the histidine content was significantly increased in the PP group (p < 0.05). In terms of texture, dietary plant protein significantly improved the muscular hardness, gumminess, and chewiness. The activities of intestinal trypsin and alkaline phosphatase (ALP) of fish in the PP group were significantly higher than that in the AP group (p < 0.05). The expression levels of hepatopancreas TOR and IGF1 genes in the PP group were significantly higher than that in the MP group (p < 0.05). The present results indicate dietary fishmeal significantly improved the growth performance and muscular flavor glycine content, while dietary plant-based protein increased crude protein content, antioxidant ability, and muscular texture performance of Triploid Crucian Carp.

Liver Kinase B1 (LKB1) is a serine/threonine kinase that can regulate energy metabolism and skeletal muscle growth. In the present study, LKB1 cDNA of triploid crucian carp (Carassius auratus) was cloned. The cDNA contains a complete open reading frame (ORF), with a length of 1326 bp, encoding 442 amino acids. Phylogenetic tree analysis showed that the LKB1 amino acid sequence of the triploid crucian carp had a high sequence similarity and identity with carp (Cyprinus carpio). Tissue expression analysis revealed that LKB1 was widely expressed in various tissues. LKB1 expressions in the brain were highest, followed by kidney and muscle. In the short-term LKB1 activator and inhibitor injection experiment, when LKB1 was activated for 72 h, expressions of myogenic differentiation (MyoD), muscle regulatory factor (MRF4), myogenic factor (MyoG) and myostatin 1 (MSTN1) were markedly elevated and the content of inosine monophosphate (IMP) in muscle was significantly increased. When LKB1 was inhibited for 72 h, expressions of MyoD, MyoG, MRF4 and MSTN1 were markedly decreased. The long-term injection experiment of the LKB1 activator revealed that, when LKB1 was activated for 15 days, its muscle fibers were significantly larger and tighter than the control group. In texture profile analysis, it showed smaller hardness and adhesion, greater elasticity and chewiness. Contrastingly, when LKB1 was inhibited for 9 days, its muscle fibers were significantly smaller, while the gap between muscle fibers was significantly larger. Texture profile analysis showed that adhesion was significantly higher than the control group. A feeding trial on triploid crucian carp showed that with dietary lysine-glutamate dipeptide concentration increasing, the expression of the LKB1 gene gradually increased and was highest when dipeptide concentration was 1.6%. These findings may provide new insights into the effects of LKB1 on fish skeletal muscle growth and muscle quality, and will provide a potential application value in improvement of aquaculture feed formula.

Kidney-type glutaminase, encoded by the gls1 gene, plays a critical role in glutamate production and improvement of meat flavor. In this study, a gls1 gene encoding 595 amino acids was cloned from triploid crucian carp (Carassius auratus) (TCC) and showed a high similarity with the gls1 gene found in Cyprinus carpio, Sinocyclocheilus rhinocerous and Puntigrus tetrazona. Comparing the abundance of gls1 in different tissues, we found its expression level in the brain and liver were significantly higher than that in heart, gut, kidney, spleen and muscle. gls1 expression in the brain reached the highest value. In addition, the expression levels of gls1 also appeared different in diurnal variation, with the highest expression seen at 9:00, while it was low at 3:00, 6:00, 15:00 and 24:00. Furthermore, dietary regulation of gls1 expression was investigated in our study. In each feeding trial, each diet was randomly assigned to triplicate tanks. Fish were fed one of the tested diets up to satiation twice daily. The results showed that gls1 expression increased in 32% protein group and decreased in 35–41% protein group. The results of different protein source experiments showed that the expression of gls1 gene in the mixed protein group (the control group) was significantly higher than that in the fish meal and soybean meal groups. Glutamate treatment revealed that appropriate concentrations (0.10 mg/mL in vivo and 2.00% in vitro) of glutamate remarkably improved the expression of gls1. Besides, diets supplemented with 0.80–1.60% lysine-glutamate dipeptide exhibited a down regulatory impact on gls1 expression. In conclusion, this study demonstrated that the expression of gls1 in TCC was increased by 32% protein diet, mixed protein source diet and diet with 2.00% glutamate concentration, while decreased by 0.80–1.60% lysine-glutamate dipeptide. The findings of this study provide a reference for the regulation of gls1 and have a potential application in the optimization of dietary formula in aquaculture.

A 54‐day feeding trial was conducted to investigate the effects of dietary protein levels on growth performance, digestive capacity, plasma biochemical indexes, and gene expression in Hefang crucian carp ( Carassius auratus ). Six experimental diets were prepared with protein levels of 21.7%, 24.0%, 29.4%, 34.0%, 38.0%, and 43.2%, respectively. Each diet treatment was randomly assigned to triplicate groups of 25 fish, with an initial body weight of (14.60 ± 0.39) g per tank. Fish were fed twice daily to apparent satiation. The results showed that weight gain rate (WGR) and specific growth rate (SGR) exhibited an upward trend with increasing dietary protein levels, and reached the highest value in the 43.2% protein group. Conversely, protein efficiency was significantly higher in the 24.0% protein group compared to the other protein groups ( p < 0.05). As dietary protein levels increased, there was a significant rise in the condition factor (CF), while both the hepatopancreas somatic index (HSI), viscerosomatic index (VSI) and crude lipid of whole body experienced significant decreases ( p < 0.05). The intestinal trypsin activity was significantly elevated in the high‐protein groups (38.0% and 43.2% protein) compared to the low‐protein group (21.7% protein), whereas the plasma triglyceride concentrations exhibited a significant inverse relationship ( p < 0.05). However, at a dietary protein level of 43.2%, the lipid droplet content in the liver of Hefang crucian carp was significantly elevated compared to other experimental groups. Additionally, the group fed the diet with a protein level of 21.7% exhibited significantly higher lipid droplet content than the groups with protein levels of 24.0% and 34.0%. A curvilinear regression analysis indicated that the lipid droplet area in the liver was minimized at a dietary protein concentration of 29.9%. Regarding intestinal gene expression, the asct2 and lat2 genes exhibited the highest expression levels in the group supplemented with 43.2% protein. Conversely, the expression levels of pept1 and cdx2 genes initially decreased and subsequently increased as the dietary protein levels rose, achieving their peak in the low‐protein groups (21.7% and 24.0% protein) ( p < 0.05). Concerning hepatopancreas genes, the expression levels of tor , igf1 , and gh genes were significantly elevated in the 24.0% protein group compared to other treatment groups. Additionally, the expression levels of ghr were significantly higher in the 21.7% and 43.2% protein groups compared to the other treatment groups ( p < 0.05). In conclusion, Hefang crucian carp exhibited optimal growth performance at a dietary protein level of 43.2%, whereas a protein concentration of 29.9% was identified as the critical threshold for maintaining optimal liver health.