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【Objective】The study was carried out to explore the effects of acute hypoxia on the survival of juvenile Eleutheronema tetradactylum and their spleen and heart tissue structure, with a view to providing technical support and theoretical references for the breeding and production of this species.【Method】Filling water with nitrogen to quickly overflow oxygen and reduce the content of dissolved oxygen in the water. Different levels of dissolved oxygen treatment were set in the experiment: normal oxygen group (6.33 ± 0.15 mg/L), moderate hypoxia group (3.99 ± 0.18 mg/L), and severe hypoxia group (2.05 ± 0.15 mg/L). Each treatment group was set up with 3 parallel devices. During the experiment, the flowing water was turned off, and the dissolved oxygen level in the water was monitored in real time by using a dissolved oxygen measuring instrument. The corresponding dissolved oxygen level was maintained by adjusting the size of the inflation valve. Using paraffin embedding, sectioning and hematoxylin eosin (HE) staining methods were used to observe the changes in tissue structure of the heart and spleen of juvenile E. tetradactylum.【Result】In terms of mortality rate, there was no mortality observed in the E. tetradactylum in the normal oxygen group; the mortality rates of juvenile fish under the stress in the moderate and severe hypoxia groups were 10.0%-13.3% at 23 hours, 30.0%-40.0% at 26 hours, and 43.3%-53.3% at 29 hours. In terms of spleen tissue structure, compared with the normal oxygen group, the moderate hypoxia group showed an increase in white pulp and the number of lymphocyte in juvenile fish, sparse distribution of peripheral red pulp, and a decrease in the number of melanin macrophages; the red pulp area of juvenile fish in the severe hypoxia group expanded, with a decrease in the white pulp area and the number of lymphocytes, and melanin macrophages rarely appearing. In terms of heart tissue structure, the compact layer of heart tissue in juvenile fish in the moderate hypoxia group became thinner, the myocardial fibers in the sponge layer became thinner, and the spacing became wider, and partial myocardial fiber rupture phenomenon occurred; the degree of thinning of the compact layer and thinning of the myocardial fibers in the sponge layer of juvenile fish in the severely hypoxic group increased, and the degree of expansion of the interstitial space increased compared to the hypoxic group, with partial myocardial cell rupture, nucleus protrusion, myocardial fiber rupture, and disordered arrangement.【Conclusion】The study has shown that acute hypoxia stress can affect the survival rate of juvenile E. tetradactylum. The lower the dissolved oxygen level, the higher the mortality rate, and the degree of tissue structure damage increases with the level of hypoxia.

【Objective】The effects of different mass concentrations of MS-222 and eugenol stresses on gill and liver tissue damage in juvenile Eleutheronema tetradactylum were studied.【Method】Seven different mass concentrations of MS-222 (20, 30, 35, 40, 45, 50 and 60 mg/L) and seven different mass concentrations of eugenol (5, 10, 20, 25, 30, 40 and 50 mg/L) were set to analyze the static anaesthesia effects of different treatments on juvenile E. tetradactylum with an average length of 4.5 (±0.5) cm and an average body weight of 1.5 (±0.5) g.【Result】When the concentration of MS-222 was not less than 35 mg/ L, the juvenile fish began to show stress reactions after 467 s of anesthesia. When the concentration was 50 mg/L, some juvenile fish could be induced into the anesthesia period. When the concentration was 60 mg/L, all juvenile fish could be induced into the anesthesia period after 293 s. When the concentration of eugenol was not less than 25 mg/L, the juvenile fish began to exhibit stress reactions after 291 s of anaesthesia; when the concentration was 30 mg/L, some juvenile fish could be induced into the anaesthesia period; when the concentrations were 40 and 50 mg/L, all juvenile fish could be induced into the anaesthesia period at 194 s and 102 s, respectively. With the increase of MS-222 and eugenol concentrations, the time for juvenile fish to enter the same anaesthesia period was significantly shortened (P < 0.05). At high concentrations, the time for juvenile fish to enter the anaesthesia period was too fast, which exceeded the tolerance value and may have a certain toxic effect on the fish body, leading to fish death. By observing the pathological sections of juvenile fish tissues, it was found that, with the increase of MS-222 and eugenol concentrations, the liver cells of juvenile fish showed vacuoles, disordered arrangement, nuclear dissolution visible in the nucleus and unclear liver plate structure. The gill lamellae were curved with enlarged ends, and they may burst or fall off in severe cases.【Conclusion】A moderate dosage of anaesthetic can reduce the stress response of fish to the external environment and reduce oxygen consumption. Different concentrations of MS-222 and eugenol (except for 5 mg/L) have a sedative effect on E. tetradactylum within 3 minutes. When the concentrations of the two anaesthetics are too high, the anaesthesia time is too long, and the juvenile fish are in a state of stress for a long time, the fish will die.