Structure and gene expression changes of the gill and liver in juvenile black porgy (Acanthopagrus schlegelii) under different salinities

Black porgy (Acanthopagrus schlegelii) is an important marine aquaculture species in China. It is an ideal object for the cultivation of low-salinity aquaculture strains in marine fish and the study of salinity tolerance mechanisms in fish because of its strong low-salinity tolerance ability. Gill is the main osmoregulatory organ in fish, and the liver plays an important role in the adaptation of the organism to stressful environments. In order to understand the coping mechanisms of the gills and livers of black porgy in different salinity environments, this study explored these organs after 30 days of culture in hypoosmotic (0.5 ppt), isosmotic (12 ppt), and normal seawater (28 ppt) at histologic, physiologic, and transcriptomic levels. The findings indicated that gill exhibited a higher number of differentially expressed genes than the liver, emphasizing the gill's heightened sensitivity to salinity changes. Protein interaction networks and enrichment analyses highlighted energy metabolism as a key regulatory focus at both 0.5 ppt and 12 ppt salinity in gills. Additionally, gills showed enrichment in ions, substance transport, and other metabolic pathways, suggesting a more direct regulatory response to salinity stress. The liver's regulatory patterns at different salinities exhibited significant distinctions, with pathways and genes related to metabolism, immunity, and antioxidants predominantly activated at 0.5 ppt, and molecular processes linked to cell proliferation taking precedence at 12 ppt salinity. Furthermore, the study revealed a reduction in the volume of the interlamellar cell mass (ILCM) of the gills, enhancing the contact area of the gill lamellae with water. At 0.5 ppt salinity, hepatic antioxidant enzyme activity increased, accompanied by oxidative stress damage. Conversely, at 12 ppt salinity, gill NKA activity significantly decreased without notable changes in liver structure. These results underscore the profound impact of salinity on gill structure and function, highlighting the crucial role of the liver in adapting to salinity environments.