Specific Features of the Ultrastructure of Mesonephros of Smolts of the Atlantic Salmon Salmo salar L. (Baltic Sea population) and Brown Trout Salmo trutta L.

The ultrastructure of the mesonephros of the natural population of Atlantic salmon smolts (Salmo salar L., 1758) and brown trout smolts (Salmo trutta L., 1758) inhabiting the Luga River in Leningrad oblast, Russia, was studied. The general plan of the mesonephric kidney structure is presented. Specific features of the structure of leukocytes and nephron of salmonids under smoltification are described. The size of leukocytes and the height of the epithelial cells of the proximal tubules of the kidneys of salmon smolts are greater in comparison with those kidney cells of freshwater teleost fishes. The number, size, and population density of mitochondria, as well as the development of a rough and smooth endoplasmic reticulum in different types of cells of the mesonephros of salmon smolts, are higher than those in freshwater fish. A similar pattern is observed for the number of vesicles in the basal part of the distal tubule of the kidneys of the salmon smolt in comparison with freshwater fish. Cells with radially arranged vesicles (CRAVs) of different ultrastructures have been found for the first time in mesonephros of Atlantic salmon. All the described features of the ultrastructure of mesonephros cells are revealed for the smolts prior their migration into sea water.