Histological investigation of pseudobranch during reproductive migration of Van Fish (Alburnus tarichi, Güldenstädt, 1814).

Lake Van is the largest lake in Turkey and one of the few soda lakes in the world. The Lake Van fish (Alburnus tarichi), an endemic carp species, has fully adapted to the soda water in this lake. As an anadromous fish, it migrates from the lake to Freshwater (FW) every year between April-July. During migration, the fish activate osmoregulatory mechanisms to maintain water and salt balance. This study examined the pseudobranch tissues of Lake Van fish from both Lake Van and Karasu Stream. Morphological analysis showed that the pseudobranch, located on both opercular valves, was covered with transparent connective tissue. Histologically, the tissue contained different cell types, including pavement, pillar cells, and erythrocytes. Higher mucous cell density were found in the stream environment, while immunohistochemical (IHC) analysis showed more Na+/K+-ATPase-positive cells in the stream fish. These results suggest that the pseudobranch plays a key role in osmoregulation, helping the fish adapt during migration.