β-glucan is widely used in aquaculture due to its immunostimulatory effects, but the specific effect and potential regulatory mechanism on largemouth bass (Micropterus salmoides) are still unclear. Here, we evaluated the effects of β-glucan on growth, resistance to Aeromonas schubertii, intestinal health, and transcriptome of largemouth bass to reveal the potential regulators, metabolic pathways, and altered differential microbiota. Four experimental diets were designed with β-glucan supplementation levels of 0 (control), 100 (LA-100), 200 (MA-200), and 300 (HA-300) mg kg^-1, and each diet was fed to largemouth bass (79.30 ± 0.50 g) in triplicate for 70 days, followed by a 3-day challenge experiment. Results showed that different β-glucan supplementations had no significant effects on growth performance and whole-body composition. Fish fed a diet with 300 mg kg^-1 β-glucan significantly increased the activity of lysozyme than those fed diets with 0 and 100 mg kg^-1 β-glucan. In addition, the survival rate of largemouth bass in β-glucan supplementation groups was significantly higher than the control group at 12- and 24-h challenge by Aeromonas schubertii. Transcriptome analysis showed that a total of 1,245 genes were differentially expressed [|log₂(fold change)| ≥1, q-value ≤0.05], including 109 immune-related differentially expressed genes (DEGs). Further analysis revealed that significantly upregulated and downregulated DEGs associated with immunity were mapped into 12 and 24 pathways, respectively. Results of intestinal microflora indicated that fish fed a diet with 300 mg kg^-1 β-glucan had higher bacterial richness and diversity as evaluated by Sobs, Chao, Ace, and Simpson indices, but no significant differences were found in the comparison groups. Furthermore, 300 mg kg^-1 β-glucan significantly increased the relative abundance of Mycoplasma and decreased Proteobacteria (mainly Escherichia-Shigella and Escherichia coli) and Bacillus anthracis in largemouth bass intestinal microflora. The findings of this study provided new insights that will be valuable in future studies to elucidate the mechanism of immunity enhancement by β-glucan.

The aim of this study was to evaluate the effects of wheat gluten as a substitute for fish meal (FM) and soy protein concentrate (SPC) in the low-fishmeal-based extruded diet in red spotted grouper Epinephelus akaara. Eight isonitrogenous (441–456 g kg⁻¹) and isocaloric (21.5–22.0 MJ kg⁻¹) diets were produced, including the control diet (R0), three diets with 33.3, 66.7, and 100% FM being replaced by a mixture of wheat gluten, wheat, and taurine (GWT) (RF1, RF2, RF3), three diets with 33.3, 66.7, and 100% SPC replaced by GWT (RS2, RS2, RS3) and one diet with 50% FM and 50% SPC replaced by GWT (RFS). Results showed that feed intake (FI), weight gain (WG), protein retention efficiency, and liver superoxide dismutase activity increased linearly, while feed conversion ratio (FCR) decreased linearly with the decrease of dietary FM. Additionally, FI, WG, and FCR significantly increased with decreasing dietary SPC. Overall, 100% FM or 61.2% SPC can be safely replaced by wheat gluten in the red-spotted grouper diet containing 20.0% FM and 21.4% SPC.

The aim of this study was to evaluate the effects of replacing fish meal (FM) or soy protein concentrate (SPC) with wheat gluten on growth performance, feed utilization, and nutrient digestibility and retention in Japanese seabass (Lateolabrax japonicus). Seven isonitrogenous (441–456 g kg⁻¹ crude protein) and isocaloric (21.5–22.0 MJ kg⁻¹ gross energy) diets were produced to replace 0%, 33.3%, 66.7% and 100% of FM or SPC with a mixture of wheat gluten, wheat, and taurine (GWT, 77.5% wheat gluten, 20.5% wheat and 2.0% taurine). The gradual replacement of protein in FM with GWT had no significant effects on feed intake, whole-body composition, and the hepatosomatic and viscerosomatic indices, but resulted in a linear decrease in the weight gain rate, feed efficiency, and retention of nitrogen, energy, and essential amino acids (Arg, His, Ile, Leu, Lys, Met, Phe, Thr, and Val). The apparent digestibility of most essential amino acids (Cys, His, Leu, Lys, and Phe) and total amino acids increased linearly. Replacement protein in SPC with GWT had no significant effects on feed intake, growth, the feed conversion ratio, whole-body composition, and the hepatosomatic index, but resulted in a linear decrease in nitrogen, energy, and Met retention; the digestibility of Cys and Met increased linearly. Overall, wheat gluten is a more effective alternative for replacing protein in SPC than FM.