In this, work some aspects of the development of the visual system of Nothobranchius guentheri at the main stages of ontogenesis were described for the first time. It was possible to establish that the formation of the visual system occurs similarly to other representatives of the order Cyprinodontiformes, but significantly differs in terms of the individual stages of embryogenesis due to the presence of diapause. In the postembryonic period, there is a further increase in the size of the fish’s eyes and head, to the proportions characteristic of adult fish. The histological structure of the eye in adult N. guentheri practically does not differ from most teleost fish living in the same environmental conditions. The study of the structure of the retina showed the heterogeneity of the thickness of the temporal and nasal areas, which indicates the predominant role of peripheral vision. Morphoanatomical measurements of the body and eyes of N. guentheri showed that their correlation was conservative. This indicates an important role of the visual system for the survival of fish in natural conditions, both for the young and adults. In individuals of the older age group, a decrease in the amount of sodium (Na) and an increase in magnesium (Mg) and calcium (Ca) were found in the eye lens. Such changes in the elemental composition of the lens can be a sign of the initial stage of cataractogenesis and disturbances in the metabolism of lens fibers as a result of aging. This allows us to propose N. guentheri as a model for studying the structure, formation, and aging of the visual and nervous systems.

In this study, the effect of three promising feed additives (chelated compounds of trace elements, butyric acid, lycopene) on changes in the culturable microbiota and histological parameters of two sections of the intestines of Danio rerio (zebrafish) was studied. The use of these feed additives can help to eliminate the deficiency of trace elements, modulate the composition of the microbiota due to the postbiotic properties of butyric acid, and reduce oxidative stress when using lycopene. Incorporation of the investigated supplements in the feed resulted in a significant change in the relative abundance of certain groups of microorganisms. The taxonomic diversity of cultured microorganisms did not differ in the anterior and posterior intestines, while there were differences in the relative abundance of these microorganisms. The most sensitive groups of microorganisms were the genera Bacillus and Serratia. A significant effect on the composition of the cultured microbiota was caused by lycopene (in all studied concentrations), leading to a significant increase in the relative abundance of Firmicutes in the anterior gut. Studies of the histological structure of the anterior and posterior guts have shown the relationship between the barrier and secretory functions of the gut and the composition of the microbiota while using butyric acid (1 and 2 g kg−1) and trace element chelated compounds (2 mg kg−1). This culture-dependent method of studying the microbiome makes it possible to assess changes in some representatives of the main groups of microorganisms (Firmicutes and Proteobacteria). Despite the incompleteness of the data obtained by the culture-dependent method, its application makes it possible to assess the bioactive properties of feed and feed additives and their impact on the microbiota involved in digestive processes.

This article discusses the pathogenic threat to humans thermophilic microorganisms encountered in recirculation aquaculture systems for growing fish. This article discusses such genera as — Escherichia, Salmonella, Staphylococcus, Vibrio, Pseudomonas, Aeromonas and some of their subspecies, the most characteristic and representing the greatest distribution and danger to humans, including golden and methicillin-resistant Staphylococcus aureus and MRSA (S. aureus and MRSA), hydrophilic aeromanad (A. hydrophilia), Pseudomonas aeruginosa, cholerogenic (V. cholerae) and non-cholerogenic (V. parahaemolyticus, V. vulnificus and others) vibrions. Their pathogenic mechanism, the diseases they cause in humans, their danger and resistance to environmental conditions, as well as their sensitivity to antimicrobials and antibiotics also considered. In addition, the topic of preventing them from entering the ultrasound system and possible preventive measures discussed.

This article presents results of the influence of three probiotic feed additive of various microbiological composition: Bacillus subtilis (VKPM B‐2335); B. subtilis (OZ‐2 VKPM‐11966) + Bacillus amyloliquefaciens (OZ‐3 VKPM‐11967); Lactobacillus acidophilus (VKPM B‐3235) on the growth and histology of the organs of the gastrointestinal tract of juvenile Oncorhynchus mykiss by morphometric parameters. These probiotic bacteria are the most commonly used in aquaculture. The effect of the probiotic feed additive led to the increase in fish growth and influenced different sections of the gastrointestinal tract. The biggest change was found in the mid intestine and the reliable difference compared with the control diet was obtained at the following parameters: lamina propria width, intraepithelial lymphocytes number of prismatic epithelium and goblet cells area. The changes in the pyloric appendages were less obvious but reported as playing an important functional role in digestion. The liver preserved normal functional structure in all series of the experiment except for the group with L. acidophilus, where hepatocyte small‐drop vacuolization was observed. That might be connected with the change of the digest activity resulting from a decrease in secretory activity of the intestinal exocrinocytes. The use of all probiotic feed additives led to a similar change in morphometric parameters in all groups, which suggests a decrease in the immune response.

In this paper, the toxic effect of a complex chelate mixture containing microelements (Fe, Mn, Zn, Se, I, Cu) on Danio rerio was investigated. Chelated compounds are used to detoxify elements in the aquatic environment, as well as feed additives for various types of farm animals. The effect of chelate complexes was studied according to the following parameters: LC50, survival in chronic experience (30 days), embryotoxicity and genotoxicity by the micronucleus test method. The established LC50 value was 2.73 mg/l, the maximum allowable concentration that does not have a negative effect on adults fish and eggs was 0.5 mg/l. The genotoxic effect on erythrocytes of fish blood was not found in the entire range of sublethal concentrations. Comparison of the data obtained on the toxicity in this study allows asserting that the toxicity of chelates is lower than that of the ionic forms of the elements, subject to their complex effect. To accurately determine the safe level of exposure to chelates, additional studies on other organisms of the hydrobiocenosis are required.