---

Morphological changes of the chloride cells and the α_1b subunit gene expression of Na⁺-K⁺-ATPase in triploid rainbow trout (70.6 g average weight) were studied upon direct transferring to 6, 12 and 18 ppt salinities. Changes in abundance, distribution pattern, and the sectioned area of the chloride cells was studied through classic histology and Na⁺ K⁺-ATPase localization was performed through immunofluorescence light microscopy using a mouse monoclonal antibody IgGα5. Gene expression of Na⁺-K⁺-ATPase α_1b subunit was studied by semi-quantitative gene expression methods.No mortality occurred among the fish in all salinities during the 10-days experimental period and treated fish kept their plasma osmolality at standard physiologic levels. All the fish also showed similar distribution pattern in their chloride cells that were distributed on filaments, between and over lamella. Histological studies confirmed some abnormal morphological changes such as lamella interruption. Immunohistochemical studies showed the highest number of the chloride cells on lamella and between lamella in 18 ppt and the maximum sectional area of the chloride cells in freshwater. Gene expression of Na⁺-K⁺-ATPase α_1b subunit had direct correlation with increasing trend of salinity. In conclusions, triploid rainbow trout was found to be adaptable to the various experimented salinities and could be recommended for rearing in brackish water.

---

In order to determine the effects of fish size and weight on its salinity tolerance, the chloride cells (CCs) immunolocalization changes were examined in Caspian salmon (Salmo trutta caspius) parrs, with the same age (about 2 years old) but different types (type A: 4.88 g, 8.36 cm; type B: 14 g, 11.84 cm; type C: 24.05 g, 14.08 cm). Fish survival rate, blood osmolality, gills CCs histological and immunohistochemical changes were investigated following their transfer from freshwater (FW) to the Caspian Sea water (CSW). The survival rate increased in larger sizes and blood osmolality showed a tendency to increase in parallel with salinity. After 10 days in CSW, some abnormalities were observed in gill structure such as: lamellae cohesion, lamellae rupture and separated lamella from filament epithelium that were shown in all types. These abnormalities in type B were less than the other types. Gill CCs were observed on the gill filament and lamellae. In direct transfer of Salmon parrs to the CSW, changes in number, sectional area, and the surface occupied by CCs in the gill tissue were observed in all the three types. In the type B, the number of CCs did not change, however, in the type C, they decreased, while in the type A, there was significant increase. But in the CSW, the occupied surface by gill CCs in the type C, was reduced significantly compared to other types. According to the present results, among the Salmon parrs with the same age, the fish with type A lacked osmoregulation, while the type B had better compatibility with the CSW. However, after reaching the size of the type C and considering osmultification, it is probable that the type B fish would become compatible with the fresh water environment and they will not have osmoregulation ability in saline water for the long term.