The dynamicity of the mesonephros microstructure in bream (Abramis brama Linnaeus, 1758) of the Dnieper-Bug estuary system in relation to spawning migration

Purpose. To analyze the mesonephros structure in the bream (Abramis brama Linnaeus, 1758) during its spawning migration, to determine the perspective of the use of microanatomical monitoring data in fisheries practice for assessing the functional status of fish. Methodology. Collection of ichthyological material was carried out using permit documents of the Institute of Fisheries NAAS of Ukraine, while their processing was done at the Department of Aquatic Bioresources and Aquaculture of the Kherson State Agricultural University. Ichthyological material was collected using gill nets, fyke nets, beach seines. Monitoring stations were located along the spawning migration run of the bream from the Dnieper lagoon to the plavni system of the Dnieper River. Field and laboratory processing of the material was done using author's equipment and original methods specially designed for histological diagnostics of tissues of aquatic animals. Findings. The basis of the adaptation of bream kidneys to waters of different salinities us structural irregularity of nephrons expressed in the size and structure of renal corpuscles and convoluted tubules. Significant fluctuations of osmolarity parameters within the lower Dnieper area have significant effects on changes in the structure and function of nephrogenic tissue in form of the reduction in the diameter of Boumen-Shumliansky capsule by 9.0 µm. Changes of the Boumen-Shumliansky capsule diameter in fish with moderate mineralization is the range of 6.0 and 4.0 µm. The reduction in the height of epitheliocytes of proximal convoluted tubules by 3.0; 2.0 and 4.0 µm also indicate on the reduction of the functional activity of the convolute. The fluctuations of nephrogenic and homeopathic tissue ratio have a balance nature that can be related to the redistribution of these components due to the appearance of an adaptation under the effect of chloride-ions. The dislocation of ion-transporting cells near the blood vessels and around tubule epitheliocytes indicate on the participation of mesonephros in ion-regulating processes by increasing the resistance to changes in the osmolarity of the medium. The functional activity of a nephron changes when the osmolarity in bream mesonephros decreases that is manifested as a reduction in the diameter of renal corpuscles and height of the epithelium of proximal convoluted tubules. Originality. The detailed microanatomical analysis of mesonephros structure of mature bream of the Dnieper-Bug mouth system has been performed for the first time. The existing information on the taxonomic peculiarities of the transformation of nephrogenic tissue of fish in the conditions of spawning migration has been significantly elaborated and updated. Practical value. The actual information gives an opportunity to use the obtained material not only in morphophysiological studies but also in the practice of ichthyology and aquaculture in relation to the existing problem of the reduction in the life status of individual fish that results in the reduction of the stock of valuable commercial species, which formed partially migratory forms in the delta areas of river ecosystems.