The effect of salinity of different habitats on the kidney tissue of Caspian kutum (Rutilus frisii ) fingerlings

The migration of fish to the sea requires the ability of osmoregulation to exposure to salinity, and changes in the structure and function of many organs, including the kidney, are involved in the developing this process. In this study, quantitative and qualitative changes in the kidney tissue of Caspian kutum (Rutilus frisii) fingerlings exposed to water salinity in the coastal and estuarine areas of the Caspian Sea were investigated. Three weighing groups 0.5, 1, and 1.5g were treated for 10 days in the three natural environments of the river (freshwater), the estuary (6-7‰), and the coast of the Caspian Sea (11-12‰) in the Kiasher beach area. At the beginning (0h) and end of the experiment (240h), 6 individuals from each weight group in each treatment were caught. After dissecting and separating the kidney tissue, the area and diameter of the glomerulus and Bowman's capsule were measured. The results showed that at the end of the experiment, the size of the kidney tissue indices significantly decreased (P<0.05) with the increase in salinity in all treatments. So that, the extent of this decrease in 1 and 1.5g weight groups was significantly more than the 0.5g group (P<0.05). Qualitative changes, including a decrease in the width of the proximal and distal convoluted tubule after exposure to salinity, were observed in all weight groups. The decrease in the size of the glomerulus in the estuary and coastal environments can be because constant drinking of water in these environments causes enough ions to be provided to the body and there is no need for large glomeruli for filtration. In the present research, two weights of 1 and 1.5g can compromise with the conditions of increased salinity of the environment when released and placed in the estuary and coastal environments of the Caspian Sea.