Effects of MS-222 and Eugenol Stress on Microstructure of Gill and Liver Tissue in Juvenile Eleutheronema tetradactylum

【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.