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Rainbow trout (Oncorhynchus mykiss) is a hypoxia-sensitive fish, and its growth, behavior, metabolism, and immunity are affected in hypoxic environments. To elucidate the effects of hypoxic stress on the heart of rainbow trout, biochemical indices and hypoxia-related gene expression were measured during moderate (4.5±0.1 mg/L) and severe hypoxia (3.0±0.1 mg/L) stress for 4 h, 8 h, 12 h, and 24 h; moderate hypoxia for 1 month (TMM); severe hypoxia for 1 month (TMM); and reoxygenation (8.5±0.1 mg/L) for 12 h and 24 h using enzyme activity assays and quantitative real time polymerase chain reaction (RT-qPCR). The results showed that the pyruvate kinase (PK), total cholesterol (TC), lactic acid (LD), and glutamine aminotransferase (GPT) reactivates increased at 8 h, decreased at 24 h, and were significantly higher than the control levels after reoxygenation (P < 0.05) under moderate hypoxic stress. Under severe hypoxic stress, the succinate dehydrogenase (SDH) activities gradually increased and peaked at 8 h (P < 0.05). No significant difference was observed between the control at 24 h and after reoxygenation (P > 0.05). The adenosine triphosphatase (ATPase), lipase (LPS), TC, glutamic transaminase (GOT), and GPT reactivates decreased at 12 h and recovered to normal levels after reoxygenation (P > 0.05). The PK, TC, lactate dehydrogenase (LDH), and GPT reactivates were significantly higher in the TMM and TMS groups than those in the control group (P < 0.05). The expressions of succinate dehydrogenase gene (sdh), factor inhibiting hypoxia-inducible factor-1 (fih1), and hypoxia-inducible factor-1α (hif-1α) were significantly increased at 8 h compared to those of the control (P < 0.05) and returned to normal reactivates after reoxygenation. Under severe hypoxic stress, lactate dehydrogenase gene (ldh), pyruvate kinase gene (pk), sdh, hif-1α, proline hydroxylase domain protein 2 (egln-1), and von Hippel-Lindau (vhl) expressions were significantly increased at 24 h (P < 0.05) compared to those under moderate hypoxic stress. Compared with those of the control group, ldh, pk, sdh, hif-1α, egln-1, and vhl expressions were significantly decreased in the TMM and TMS groups under moderate hypoxic stress with no significant differences (P > 0.05). pk, sdh, and vhl were significantly increased under severe hypoxic stress (P < 0.05). This study indicated that varying levels of dissolved oxygen led to changes in biochemical indices and in the expression of hypoxic-related genes in the heart of rainbow trout. Hypoxic stress affected the cardiac metabolism of rainbow trout, which affected the normal metabolism level and caused damage. Under hypoxic stress, the rainbow trout were able to provide feedback regulation of hypoxic stress through the high expression pattern of hypoxic-related, which prevents the heart from being in a constant state of hypoxic stress, allowing the organism to quickly return to a stable state and perform its normal physiological function. This study provides basic data to further elucidate the regulatory mechanism of cardiac metabolism in rainbow trout under hypoxic stress, it has guiding significance for the intensive and healthy breeding of this fish and the selection and breeding of new hypoxia-tolerant species in the future.

Exopalaemon carinicauda has the highest production in the Yellow Sea and Bohai Sea of China. Compared with those of other shrimp species, E. carinicauda has a short reproductive cycle, fast growth, and adaptability. The pond mono-culture and mixed culture mode is more common in small economic shrimp and can be used as an excellent biological material for crustacean biology research. Dissolved oxygen, an important environmental factor for aquatic animal survival, affects the growth, behavior, reproduction, immunity, and metabolism of aquatic animals and is highly susceptible to hypoxia due to high temperatures, flushing, water pollution, the expansion of the scale of aquaculture, high-density aquaculture, high temperatures, and climate change. Recently, studies have been conducted on the effects of hypoxia on aquatic animals, mainly focusing on immune response, energy consumption, respiration, and antioxidant character. Gill and hepatopancreas tissue are major organs in crustaceans and increasing evidence reveal that gut bacterial community are involved in host immune defense, nutrient absorption, and antioxidant processes. Therefore, investigating the effects of hypoxic stress on the tissue structure and gut bacterial community of E. carinicauda will help to elucidate the mechanism of the response of E. carinicauda to hypoxic environments, which will be useful for its healthy aquaculture. In this study, we collected gill, hepatopancreas, and gut tissues from experimental and control groups after 24 h of hypoxic stress (2.5±0.2) mg/L. The gills and hepatopancreas were sectioned and observed and the changes in gut bacterial community before and after hypoxic stress were analyzed using 16S rRNA gene sequencing. The experimental results showed that gill and hepatopancreas tissues underwent different degrees of damage after hypoxic stress; the gut bacterial community changed, some pillar and epithelial cells of gill tissues were disordered, the number of chloride cells was significantly reduced, chloride cells were changed from irregularly flattened to rounded, the secondary lamellae was aggravated, and the gill tissues changed their morphology and structure to alleviate the hypoxic stress. The number of storage cells in hepatopancreas tissues did not change significantly compared with that of the control group, the lumen contracted significantly, the morphology and structure of the entire hepatic tubule contracted, the volume of transport vacuole increased significantly and even ruptured, and the number of secretory cells decreased significantly. Although shrimp gut bacteria are numerous and diverse the vast majority of gut bacteria are reported to be concentrated in a few dominant bacterial phyla, such as Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes. The richness and diversity of gut bacterial community of individuals in the experimental group changed significantly after hypoxic stress and Proteobacteria and Firmicutes were the dominant bacterial phylum in the guts of the control group, accounting for 81.42% and 11.18% of the total amount, respectively. The amounts of Bacteroidota and Actinobacteriota were significantly higher (P < 0.05), and that of Proteobacteria were significantly lower (P < 0.05) in the experimental group. At the family level, the numbers of Prevotellaceae and Lachnospiraceae were significantly higher (P < 0.05) in the experimental group than those in the control group. Rhodobacteraceae bacteria had a relative high abundance in the gut of healthy E. carinicauda and their amounts were significantly reduced after hypoxic stress. In addition, the numbers of some potential pathogenic bacteria were significantly higher (P < 0.05) in the experimental group. At present, multiple studies are being conducted on aquatic animals under hypoxic stress; however, the effects of hypoxic stress on the tissue structure and intestinal flora of E. carinicauda remains unexplored. In this study, we observed the damage of tissue structure and simultaneously analyzed the changes in intestinal flora of E. carinicauda under hypoxic stress, to deeply study the physiological response to stress under hypoxic conditions of E. carinicauda and provide basic scientific research on the actual production and cultivation of a novel species of E. carinicauda that is resistant to hypoxic conditions. This will provide basic scientific research information for the actual production and cultivation of novel hypoxia-tolerant varieties of E. carinicauda.

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Copyright of South China Fisheries Science is the property of South China Fisheries Science Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)