Sexual dimorphism and microanatomical parameters of the Black sea sprat Clupeonella cultriventris (Nordmann, 1840) of the Bug lagoon during its spawning migration

Purpose. To analyze sexual dimorphism of the Black Sea sprat Clupeonella cultriventris (Nordmann, 1840) of the Bug lagoon under current conditions. To study the dynamics of changes in mesonephros structure during spawning migration from the estuary to the Southern Bug River. To determine the prospect of using the data of histological monitoring in ichthyological practice for the assessment of fish morphofunctional status. Methodology. Primary materials were obtained during scientific and commercial fishery surveys carried out in the Dnieper-Bug estuary system. Control stations were located along the spawning migration pathways of the Black Sea sprat from the Bug lagoon to the South Bug River. Field and laboratory processing of ichthyological samples was performed according to generally accepted methods and guidelines. Histological studies were performed using author's equipment and original methods specially intended for fish tissue diagnostics. Findings. The studies indicated on the presence of a significant sexual dimorphism in the Black Sea sprat Clupeonella cultriventris (Nordmann, 1840) of the Bug lagoon. The comparative analysis of twenty-four plastic features showed that sprat females had a higher body depth (td=6.11; р<0.05), shorter body (td=8.62; р<0.05)and caudal peduncle (td=7.64; р<0.05). Females also had the displacement of fins in the caudal direction that that was manifested as a significant difference between antedorsal, antepectral, anteventral, anteanal, pectoventral, and ventroanal distances (td=4.39-9.67; р<0.05). Linear parameters of fins (length, height) also differed significantly (td=3.96-11.23; р<0.05). It is important to mention that these features in females were larger than those in males. An exception was the anal fin height, which was significantly higher in males than in females (td=6.85; р<0.05). No differences were detected for abdominal fin length (td=0.03; р<0.05) that was also typical for head length (td=0.15; р<0.05). No significant differences were detected in the head area. A significant differences was observed only for eye diameter and postorbital distance (td=4.15-7.99; р<0.05). Studies of microanatomical feature showed that osmolarity fluctuations of aquatic environment had a significant effect on changes in the histological structure and functional peculiarities of the nephrogenic tissue through the reduction in the diameter of Bowmen-Shumlyansky's capsule by 7.0 µm. The reduction in the functional activity of renal convolute was demonstrated by the reduction in the height of cubic epithelium cells of convoluted tubules by 3.0 µm. Localization of ion-transporting cell groups near vascular elements and tubular epithelium indicate on the participation of mesonephros in ion-regulating processes that increase the resistance of fish to the changes in water salinity. If osmolarity of aquatic environment reduces, functional nephron activity in sprat kidney changes that is manifested as a reduction in the diameter of renal bodies and height of the epithelium of proximal convoluted tubules. Originality. The paper contains new and significantly updated already existing data on the structure of the Black Sea spat spawning stock of the Dnieper-Bug estuary system. A histological analysis of the mesonephros structure of mature Black Sea sprat was performed for the first time. Information on taxonomic peculiarities of adaptation changes in the nephrogenic tissue in the condition of spawning migration was specified and complemented. Practical value. The obtained information is valuable for morpho-physiological studies as well as for ichthyological practice due to the existing current problem of the decrease in the vital status of sprat because of the reduction in the stocks of a number of valuable commercial fish species in delta areas of the river systems of Ukraine.