This article presents data on the mesonephros histology and ultrastructure in the Atlantic salmon from the Baltic Sea and Barents Sea populations, with an emphasis on comparisons between the following ontogenetic stages: parr, smolting, adult life at sea, the adults’ return to their natal river to spawn, and spawning. The ultrastructural changes in the renal corpuscle and cells of the proximal tubules of the nephron occurred as early as the smolting stage. Such changes reflect fundamental alterations during the pre-adaptation to life in saltwater. In the Barents Sea population, the adult salmon sampled in the sea had the smallest diameters of the renal corpuscle and proximal and distal tubules, the most narrow urinary space, and the thickest basement membrane. In the group of salmon that entered the mouth of the river and spent less than 24 h in freshwater, the structural rearrangements occurred only in the distal tubules. Better development of the smooth endoplasmic reticulum and a greater abundance of mitochondria in the tubule cells were observed in the adult salmon from the Barents Sea compared to those from the Baltic Sea. Cell-immunity activation was initiated during the parr–smolt transformation. Another pronounced innate-immunity response was registered in the adults returning to the river to spawn.

The study presents new data on the structural and functional organization of the mesonephros of the grayling Thymallus thymallus (Linnaeus, 1758). Adult grayling were sampled in the middle course of the Unya River, a tributary of the Pechora River (Komi Republic, Russia). The mesonephros of the grayling, as of other freshwater fishes, is composed by nephrons, blood vessels and hematopoietic tissue forming the renal interstice. In the interstice, сells with a radial vesicle array and chloride cells were discovered; the latter were mostly localized near the renal tubules. The degree of the interstice development in the mesonephros of the grayling was determined. New data on the ultrastructure of leukocytes, cells with a radial vesicle array, chloride cells, and nephron segments were obtained. A lack of mesangial cells, a small number of podocytes, and a thin basement membrane were observed on the sections of a renal corpuscle, being characteristic features of the ultrathin organization of the mesonephros in freshwater members of Salmoniformes and Esociformes. In the grayling’s nephrons, no neck segment was found, which was reported earlier for several species, including mammals. On the sections of proximal tubules, the ciliated cells were rare, and large amounts of the tubular-vesicular network in the zone of endocytosis of the type II epithelial cells were observed. On the sections of distal tubules, short blade-shaped cytoplasmic processes, with large numbers of invaginations of cytoplasmic membrane, were found. On the basis of the distinctive ultrastructure features mentioned above, the inference that grayling show the cytological markers of adaptation to euryhalinity was made. Thus, the results contribute to the knowledge of mesonephros development in fishes during their life history. From the species protection standpoint, our study provides baseline data on a WBC differential in the mesonephros as well as superoxide dismutase, catalase, glutathione S-transferase, and ethoxyresorufin-O-deethylase activities, which can be used in further studies addressing the health status of grayling populations.

We researched the influence of the additive Chlorella cultivated in a closed bioreactor under the influence of an electrostatistic field, on the biochemical and hematological parameters of blood in different age groups of cattle. The experiment was conducted on two groups of three month old heifers and two groups of lactating cows in the 7th month of lactation, with 15 individuals in each group; all animals in the groups were given the basic diet. During the 120-day experiment, the experimental group of heifers was fed on a suspension of Chlorella cultivated with the technology using an electrostatic field, in the following amounts: 1–30th day – 1.51 g, 31–60th day – 1.82 g, 61–90th day – 2.01 g, 91–120th day – 2.28 g of dry substance per individual daily; the group of lactating cows during 60 days was given the Chlorella additive to the amount of 6 g of dry substance per individual per day. Over the following 30 days the heifers and lactating cows were kept under observation to determine the “aftereffects” of the Chlorella suspension fodder additive. On the first day of the experiment and after every 30 days, blood was drawn from the animals’ jugular veins for determination of the biochemical and hematological parameters of the blood During the experiment, the experimental groups of heifers and lactating cows were observed to have a strengthened antioxidant system due to increase in the activity of superoxide dismutase and catalase, and also to have a strengthened hematopoietic function and intensified metabolic and redox processes. Due to the stimulating effect of the suspension of Chlorella cultivated using the intense technology involving using an electrostatic field on the erythro- and leukopoiesis of different age groups of the cattle, we determined a strengthening of the cellular element of the animals’ immune system. During the following 30 days, the heifers and lactating cows of the experimental groups were observed to exhibit “aftereffects” of the Chlorella suspension additive, namely increases in the values of biochemical, hematological and morphological indicators of the blood in comparison with these indicators for the animals in the control groups. As a result, we concluded that feeding different age groups of cattle with suspension of Chlorella cultivated in the conditions of a closed bioreactor under the influence of an electrostatic field, in the dosage of 155 ml per kg of dry substance of the animals’ diet, contributes to the strengthening of non-specific defence of their organism, forming a potential for increasing the productive indicators and maintenance of the livestock.

The task of optimizing the quality of feed and, hence, the rate of biomass accumulation by fish in aquaculture requires improved knowledge on the activity of their digestive enzymes and the composition of intestinal microbiota. This is the first study of the activity of glycosidases and proteinases in the intestinal mucosa and chyme, as well as of the intestinal microbiome composition, in the African catfish Clarias gariepinus grown in tanks with a closed water supply system. The total amylolytic and total proteolytic activities in the intestinal chyme were significantly higher than those in the mucosa, and higher in small fish than in large ones. The activity of disaccharidases (sucrase and maltase) in the intestinal mucosa did not depend on the fish size. In both size-age groups of African catfish, the following bacterial phyla were found to be dominant: Pseudomonadota, Bacillota, Actinomycetota, Fusobacteriota, and Bacteroidota. In the group of small fish, Pseudomonadota accounted for almost half of the number of intestinal bacteria, whereas in large catfish, the distribution of these bacteria phyla was more uniform. For the first time, the relationship between the activity of digestive enzymes and the relative abundance of bacteria of various taxa was revealed. The highest positive correlation in the chyme of small catfish was found for the total proteolytic activity with abundance of bacteria of the order Lactobacillales of the class Bacilli. In large catfish, the highest positive correlations were established for the total proteolytic activity of the intestinal chyme and abundance of bacteria of the orders Actinomycetales and Bacillales, and for the total amylolytic activity, with abundance of bacteria of the orders Fusobacteriales and Clostridiales (Eubacteriales). The data obtained can be used when developing nutritional plans for catfish in aquaculture.

The article presents the study results of the chemical composition of Cyprinidae and Percidae skeletal muscles from the Sot', Vopsha and Kast' rivers in the territory of the Yaroslavsky State Nature Reserve. The fish were ascertained for sex, stage of gonad maturity, and age. The total amount of water, dry matter, total lipids, protein, minerals, and nitrogen-free extractives (NFE) were determined in their muscle tissues. The study revealed significant differences in the content of final metabolic products in the muscles of individuals of the same species living in different rivers, which may be connected to the difference in food supply or with characteristics of hydrochemical composition of the reservoirs under study. It has been stated that the chemical composition of muscle tissues depends on the trophic status of aquatic organisms, their sex and age. On average, in the rivers the carpal skeletal muscles were superior to the perch muscular tissue in terms of total lipid content, but inferior to the amount of protein, ash, and NFE. As it was revealed, the female muscles contained more protein and fat, than muscles of males and juveniles. Reduction of dry matter and fat in the muscle tissues of roach, sabrefish and rudd (aged 4 to 5) can be associated with attaining puberty in fish. With age, a decrease of water was observed in Cyprinidae and Percidae muscles due to accumulation of dry matter. With ageing of the studied fish in their muscle tissues was found both the increased and decreased concentration of protein, total lipids, mineral substances and NFE. From the data obtained it can be inferred that the fish population of the rivers flowing through the territory of the Yaroslavsky State Nature Reserve exist in relatively favorable conditions.

The complex effect of two factors (the optimum ratio of nutrients of the medium and the electrostatic field) on the qualitative and quantitative indicators of Chlorella vulgaris was investigated. It is shown that the use of a medium with optimal concentrations of mineral substances and an electrostatic field with a voltage of 15 kV and an exposure time of 72 hours has a positive effect on growth rates, the size of microalgae cells and their viability, contributes to obtaining a culture with a density of 50 mln cells / ml 18 hours faster in comparison with the cultivation of generally accepted methods. It has been established that when two factors are influences on the cells of the Chlorella a mutagenic effect is not observed. Analysis of the microalgae culture grown under the influence of the studied factors showed that its chemical composition was consistent and with several indicators exceeded that of chlorella cultivation according to classical technologies. It was revealed that the activity of catalase and superoxide dismutase of microalgae grown in optimal conditions of the nutrient medium and the electrostatic field, significantly exceeded similar values of Chlorella vulgaris, which was not subjected to electrostatic stimulation. The high activity of the studied enzymes in Chlorella cells is shown in comparison with higher plants – components of bio-antioxidant preparations, such as Amaranthus paniculatus L. and Nicotiana tabacum L. According to the results of the toxicological study the absence of the content in Chlorella of the main poisonous substances for body animals and humans (mercury, arsenic) has been established. As a result of the research, it was proposed to use the established optimal parameters of both factors in the cultivation of C. vulgaris.