Comparison of osmotic adjustment ability of juvenile Acipenser sinensis and Acipenser dabryanus cultured in freshwater

Chinese sturgeon and Yangtze sturgeon are flagship species in the Yangtze River Basin. Due to the severe decline of natural resources, establishing artificial conservation populations and breeding and releasing them have become key measures for repairing wild populations. Chinese sturgeon and Yangtze sturgeon belong to the sisters species in the evolutionary relationship. As an important feature of species differentiation, the difference in natural life history suggests that there may be a huge difference in the osmotic pressure regulation mechanism of salinity adaptation between the two at the juvenile stage. The migratory habits of sturgeons give them unique living habits in rivers and oceans. Studying the osmotic regulation of sturgeons is of great significance for understanding their physiological characteristics and protecting this rare species.In order to investigate the difference of osmotic adjustment ability of juvenile Chinese sturgeon and Yangtze sturgeon in adapting to seawater. Freshwater cultured juvenile Chinese sturgeon and Yangtze sturgeon as research subjects, and the seawater domestication experiment was carried out by continuous salt elevation. The osmotic pressure, Na⁺ content, cortisol, gill Na⁺/K⁺-ATPase (NKA) activity and gill tissue structure changes of juvenile Chinese sturgeon and Yangtze sturgeon were compared and analyzed at 0, 9, 19, 28 during acculturation. The results of serum indexes showed that the serum osmotic pressure of juvenile Chinese sturgeon and Yangtze sturgeon increased with the increase of environmental salinity, but the serum osmotic pressure of Chinese sturgeon was significantly lower than that of Yangtze sturgeon at high salinity. Similarly, the serum Na⁺ content of Chinese sturgeon was significantly lower than that of Yangtze sturgeon. The serum cortisol content of juvenile Chinese sturgeon increased significantly, while that of Yangtze sturgeon had no significant change. Gill NKA test results showed that the gill NKA activity of Chinese sturgeon increased significantly at high salinity, and that of Yangtze sturgeon increased at 9 ppt, but decreased significantly at high salinity. H.E staining of gill tissue showed that the diameter and number of secreting chlorine cells of juvenile Chinese and Yangtze sturgeon increased significantly with the increase of salinity. However, under the condition of hyperosmosis, the gill filaments of juvenile Yangtze River sturgeon showed complete loss of structure, and the flat epithelial cells of gill lamella showed different degrees of shedding or separation. The results showed that juvenile Chinese sturgeon had strong osmotic regulation and adaptability in a high salinity environment. The body could enhance gill filament NKA activity through the regulation of hormone levels, stimulate the increase in the diameter and number of gill epithelial chlorine cells, and achieve ion exhalation and osmotic pressure balance in a high salinity environment. However, the salinity adaptation range of juvenile Yangtze sturgeon is poor and when the salinity exceeds a certain tolerance range, it will cause irreversible effects on the organizational structure. The findings provided basic data for the exploration of land-sea relay protection model of Chinese sturgeon.