This study was conducted to investigate the effects of Tenebrio molitor meal (TM) replacement for fish meal (FM) on growth performance, humoral immunity, and intestinal health of juvenile large yellow croakers (Larimichthys crocea). Four experimental diets were formulated by replacing FM with TM at different levels—0% (TM0), 15% (TM15), 30% (TM30), and 45% (TM45). Triplicate groups of juveniles (initial weight = 11.80 ± 0.02 g) were fed the test diets to apparent satiation two times daily for eight weeks. There was no significant difference in final body weight (FBW) and weight gain rate (WG) among TM0, TM15, and TM30, while TM45 feeding significantly reduced the FBW and WG. Compared with TM0, AKP activity in serum was significantly decreased in TM45, while the TM15 group remarkably increased LZM activity. TM30 showed significantly higher serum C3 levels compared to the TM0 group, while the TM addition groups decreased the C4 levels significantly in the serum. In terms of intestinal histology, the addition of TM increased the height and thickness of the intestinal villus and also increased the thickness of the intestinal muscles significantly. The addition of TM significantly reduced the serum DAO and D-lactate concentrations. The results of 16S rRNA gene sequencing showed that the addition of TM significantly enhanced the relative abundance of Bacilli and Lactobacillus and contributed to the decrease in the relative abundance of Plesiomonas. In addition, the TM30 and TM45 groups significantly reduced the abundance of Peptostreptococcaceae. Overall, our results indicated that TM could be a viable alternative protein source, 6.7% TM supplantation (replacing 15% FM) in large yellow croaker feed improved humoral immunity and intestinal health with no adverse effects on growth. Furthermore, the replacement of FM with 30% and 45% TM adversely affects growth and humoral immunity.

This study was conducted to investigate the effects of Clostridiumautoethanogenum protein (CAP) replacement for fish meal (FM) on growth performance, digestive enzyme activity, humoral immunity and liver and intestinal health in large yellow croakers (Larimichthys crocea). Four experimental diets were formulated by replacing FM with CAP at different levels—0% (CAP0), 15% (CAP15), 30% (CAP30) and 45% (CAP45). Triplicate groups of juveniles (initial weight = 11.86 ± 0.13 g) were fed the test diets to apparent satiation two times daily for eight weeks. There was no significant difference in final body weight (FBW), weight gain rate (WG) and feed efficiency (FE) between CAP0 and CAP15. However, compared to the CAP0, CAP30 and CAP45 significantly reduced FBW, WG and LDR (p < 0.05), while CAP45 significantly reduced FE and PDR (p < 0.05). The whole-body moisture was significantly increased by CAP replacement of FM while crude lipid content was decreased (p < 0.05). No significant difference in crude protein, ash and liver crude lipid was observed among all groups (p > 0.05). Compared to CAP0, CAP30 and CAP45 significantly reduced serum C4 concentration (p < 0.05), and CAP45 significantly reduced serum AKP activity (p < 0.05) but significantly increased LZM activity (p < 0.05). Serum C3 concentration was significantly increased by CAP15 (p < 0.05). In terms of intestinal histology, CAP addition significantly increased the thickness of intestinal villus (p < 0.05), and CAP15 and CAP45 significantly increased the thickness of intestinal muscular (p < 0.05). The addition of CAP significantly reduced serum DAO and D-lactate concentrations (p < 0.05), indicating the intestinal physical barrier was improved. The results of 16S rRNA gene sequencing showed that the intestinal microorganisms of large yellow croakers are dominated by organisms from Proteobacteria, Bacteroidetes and Firmicutes. The addition of CAP reduced the relative abundance of Ralstonia and Christensenellaceae and increased the relative abundance of Paenibacillus. Overall, the optimum level of CAP replacement FM in large yellow croakers feed is 15%, which helps to improve humoral immunity and intestinal health with no adverse effects on growth. However, the 30% and 45% substitution levels adversely affect the growth and humoral immunity of large yellow croakers.

Excessive fat accumulation is a common phenomenon in cultured fish, which can cause metabolic disease such as fatty liver. However, the relative regulatory approach remains to be explored. Based on this, two feeding trials were conducted. Firstly, fish were fed either a normal-fat diet (NFD) or a high-fat diet (HFD) for eight weeks and sampled at the 2nd, 4th, 6th, and 8th week after feeding (Experiment I). In the first four weeks, fish fed an HFD grew faster than those fed an NFD. Conversely, the body weight and weight gain were higher in the NFD group at the 6th and 8th weeks. Under light and transmission electron microscopes, fat accumulation of the liver was accompanied by an obvious endoplasmic reticulum (ER) swell. Accordingly, the expressions of atf-6, ire-1, perk, eif-2α, atf-4, grp78, and chop showed that ER stress was activated at the 6th and 8th weeks. In Experiment II, 50 mg/kg 4-PBA (an ERs inhibitor) was supplemented to an HFD; this was named the 4-PBA group. Then, fish was fed with an NFD, an HFD, and a 4-PBA diet for eight weeks. As the result, the excessive fat deposition caused by an HFD was reversed by 4-PBA. The expression of ER stress-related proteins CHOP and GRP78 was down-regulated by 4-PBA, and the transmission electron microscope images also showed that 4-PBA alleviated ER stress induced by the feeding of an HFD. Furthermore, 4-PBA administration down-regulated SREBP-1C/ACC/FAS, the critical pathways of fat synthesis. In conclusion, the results confirmed that ER stress plays a contributor role in the fat deposition by activating the SREBP-1C/ACC/FAS pathway. 4-PBA as an ER stress inhibitor could reduce fat deposition caused by an HFD via regulating ER stress.

Nitrite is a common pollutant in aquaculture systems and can pose serious threats to fish health, especially under high-temperature conditions. This study aimed to investigate the impact of nitrite stress on the growth, glycolipid metabolism, and hepatic metabolomic profiles in the spotted seabass fry (Lateolabrax maculatus) under elevated temperature conditions at 33 °C. A total of 450 fish (28.52 ± 0.84 g) were randomly distributed into nine tanks and exposed to three nitrite concentrations (0, 8, and 16 mg/L), with samples collected on days 1, 3, 7, 14, 21, and 28. Results showed that higher nitrite levels significantly reduced final body weight, weight gain, survival rate, hepatosomatic index, and viscerosomatic index. Blood glucose and triglyceride levels, whole-body crude lipid, liver total cholesterol, and hepatic glycogen content also declined significantly under higher nitrite stress. In contrast, hepatic lactate and lactate dehydrogenase increased in the high-nitrite group. Gene expression analysis revealed suppressed lipid synthesis and enhanced lipolysis under nitrite exposure. Metabolomic analysis further demonstrated disruptions in key energy-related pathways, including the TCA cycle, pentose phosphate pathway, and insulin signaling. These findings indicate that nitrite stress impairs growth and energy metabolism in spotted seabass, which respond by mobilizing energy reserves to cope with combined stress of high temperature and nitrite.