Effects of Isaria cicadae on growth, gut microbiota, and metabolome of Larimichthys crocea.

The large yellow croaker (Larimichthys crocea) is the most productive mariculture fish in China, and its aquaculture scale is expanding along the southeastern coast of China, but that development is causing environmental damage by increasing the use of antibiotics and other chemicals. How to improve fish immunity through non-antibiotic substances is still a problem facing aquaculture industry. At present, the experiments have shown that Isaria cicadae spent substrate (IC) can improve the growth performance and immunity of Oreochromis niloticus. Therefore, I. cicadae may be a natural alternative to antibiotic for aquaculture. In order to study the effects of IC on growth performance, serum biochemical indices, intestinal microbiota, and intestinal metabolism of large yellow croakers, the fish were divided into three groups with three replicates in each group. Basal diet, basal diet with 2% and 6% IC supplementation (IC2 and IC6 groups), respectively. The results showed that weight gain rate (WG) and specific growth rate (SGR) of large yellow croaker significantly increased (P < 0.05) in IC6 group. The content of triglyceride (TG), low density lipoprotein cholesterol (LDL-C), total protein (TP) and albumin (ALB) increased significantly (P < 0.05), and total cholesterol (T-CHO) decreased significantly (P < 0.05) in IC2 group. Compared to IC0 group, the activity of malondialdehyde (MDA) , superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) increased significantly (P < 0.05) in IC2 group, the activity of total antioxidant capacity (T-AOC) and GSH-Px increased significantly (P < 0.05) in IC6 group, and the activity of lysozyme (LZM) increased significantly in IC2 and IC6 groups. The addition of IC in the diets significantly increased the diversity of the microbial community in the intestine of large yellow croaker (P < 0.05), significantly improved the relative abundance of Acidobacteriota (P < 0.05) at the phylum level, and reduced the relative abundance of Bacteroidota, Desulfobacterota, and Synergistota (P < 0.05). At the genus level, the relative abundance of Bacteroides, Cetobacterium and Mycoplasma, which are dominant bacteria in fish gut, significantly increased (P < 0.05). The relative abundance of Ruminofilibacter, Desulfomicrobium, DMER64, Syntrophomonas, Hydrogenophaga, and Aminobacterium reduced significantly (P < 0.05). Among them, Ruminofilibacter, DMER64, Syntrophomonas and Hydrogenophaga are bacteria that can participate in the hydrolysis and acidification of organic matter, while DMER64 is the hydrogen carrier. The intestinal metabolome analysis showed that IC could improve metabolic composition and function, which was related to host immunity and metabolism. In conclusion, I. cicadae can improve the growth performance, regulate the lipid metabolism and immune and antioxidant capacity of large yellow croakers by regulating intestinal microbiota and intestinal metabolism. This study provides a reference for the application of IC in aquaculture.

Effects of constant flow velocity on endurance swimming and fatigue metabolism in red drum and blackhead seabream.

Aquaculture has greater potential for seafood production than wild capture fisheries. To meet the growing demand for seafood, China's marine aquaculture industry has begun building deep-water cages in the open sea. However, under these conditions, fish encounter strong currents and waves, and ensuring their healthy growth is key to the farming process. To address these issues, it is necessary to study the sustained swimming abilities of cultured fish species. Blackhead seabream (Acanthopagrus schlegelii) and red drum (Sciaenops ocellatus) are traditional economic fish species in China; however, their sustained swimming ability under a constant current has been underexplored. Therefore, we examined the endurance swimming ability of three size classes of blackhead seabream and red drum at 20 °C. The fish were then subjected to swimming tests of 0, 30, 60, 90, 120, and 150 min at a constant swimming speed of 0.55 m/s (0.80 m/s), 0.65 m/s (0.90 m/s), and 0.70 m/s (0.98 m/s). The fish were then dissected to obtain muscle, blood, and liver samples; sample metabolite concentrations were measured at six time points, each of which guaranteed five sets of valid data. The results indicated that red drum has a significantly stronger swimming ability, and can be cultured in waters with a short-term flow rate not exceeding 0.75 m/s or 3.5 BL/s. Further, blackhead seabream can be cultured in waters with a flow velocity lower than 0.55 m/s or 2.5 BL/s. The species-related metabolic differences were mainly reflected in the hepatic glycogen and blood glucose concentrations, and those in swimming ability caused by body length were mainly reflected by the hepatic glycogen concentration. The hepatic glycogen concentration had the most significant effect on fish with body lengths >28 cm (P < 0.05). Overall, the experimental results indicated that the liver plays a major role in the physiological level of fish swimming fatigue, providing a direction for further research.