Comparative analysis of osmoregulatory capacity in freshwater-cultured juveniles of Acipenser sinensis and A. dabryanus

Acipenser sinensis and Acipenser dabryanus 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. A. sinensis and A. dabryanus 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 A. sinensis and A. dabryanus in adapting to seawater. Freshwater cultured juvenile A. sinensis and A. dabryanus 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 A. sinensis and A. dabryanus were compared and analyzed at 0, 9, 19, 28 during acculturation. The results of serum indexes showed that the serum osmotic pressure of juvenile A. sinensis and A. dabryanus increased with the increase of environmental salinity, but the serum osmotic pressure of A. sinensis was significantly lower than that of A. dabryanus at high salinity. Similarly, the serum Na⁺ content of A. sinensis was significantly lower than that of A. dabryanus. The serum cortisol content of juvenile A. sinensis increased significantly, while that of A. dabryanus had no significant change. Gill NKA test results showed that the gill NKA activity of A. sinensis increased significantly at high salinity, and that of A. dabryanus 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 A. dabryanus 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 A. sinensis 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 A. dabryanus 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 A. sinensis.