Objective To investigate the effects of acute fresh water stress on the morphological changes of gill, heart, spleen and liver of juvenile Eleutheronema tetradactylum, in order to provide reference for exploring low salinity culture techniques and selection of good breeds.

Methods Juvenile E. tetradactylum, temporarily reared at salinity 9, with an average overall length of 2.31 cm and an average body mass of 3.81 g, were placed directly into an aquarium with a salinity of 0. Live samples were taken at 2, 6, 12, 24, 48 and 72 h. Histological methods were used to study the gill, heart, spleen and liver structures of juvenile E. tetradactylum under different durations of acute fresh water stress.

Results In juvenile E. tetradactylum, the length of the gill lamellae tended to increase gradually, the spacing of the gill lamellae increased and then decreased, the width of the gill lamellae decreased and then increased, and the number of mitochondria-rich cells continued to increase when exposed to acute freshwater stress. The number of mitochondria-rich cells continued to increase. The size of the myocardial transverse muscle, the size of the myocardial cell gap and the size of the interstitial vessels all showed a trend of increasing and then decreasing. The peak of myocardial transverse muscle size was 12 h, and the peak of myocardial interstitial and interstitial vessel size was 24 h. The white marrow area and marginal zone area gradually increased from 0 to 24 h, and the colour of melanin macrophages also gradually deepened, reaching a peak at 24 h and then gradually decreasing. 72 h later, the red marrow, marginal zone, white marrow and melanin macrophages were roughly restored to the level of the control group. In the control group, the hepatocytes were large, polyhedral, with rounded nuclei and centrally located, and some hepatocytes had 2-3 nuclei; the hepatic plate was clearly defined; the hepatic blood sinusoids were normally distributed among the hepatocytes. During 24h of stress, the hepatic plate structure was gradually blurred and the gap between the hepatic blood sinusoids was gradually widened, reaching a peak at 24h of stress; during 24-72h of stress, the hepatic plate structure gradually regained clarity, the gap between the hepatic blood sinusoids was gradually reduced and some nuclei lost their intrinsic morphology.

Conclusion Some tissues of juvenile E. tetradactylum showed stress reaction during 0-24 h, but adapted to the low-salt environment after 24 h. However, there were still some tissues that had difficulty in recovering to the control level after being subjected to low salt stress. This indicates that juvenile E. tetradactylum have some ability to adapt to low salt stress.