Morphological parameters of the digestive system of Russian sturgeon (Acipenser gueldenstaedtii Brandt & Ratzeburg, 1833) fry under exposure to dry baker's yeasts as an additive to the basic feed

Purpose. To analyze the effect of different concentrations of dry inactivated baker's yeasts on the morphological parameters of the digestive system of Russian sturgeon fry, using enterocytes and hepatocytes as an example, to determine the optimal concentrations that could be used as a part of starter feeds when growing the above mentioned fish species in recirculating aquaculture systems (RAS). Methodology. Classical histological methods were used in the study. Russian sturgeon fry were sampled every 7 days from the beginning of experimental feeding. After anesthesia, they were placed in a Buen solution (15 parts of a saturated aqueous solution of picric acid, 5 parts of formalin, 1 part of acetic acid) for 24 hours. After rinsing, the specimens were stored in 70% ethanol (С2H5OH). Specimens were processed through alcohols and embedded in paraffin in accordance with the generally accepted technique. Histologic sections were stained with hematoxylin (С16Н14О6•3Н2О, М. w. 356.33) and eosin (C20H8Br4O5, М. w. ‎647.89052). Findings. The study demonstrated the effect of different concentrations of baker's yeasts in the composition of the starter feed for Russian sturgeon on the morphometric parameters of their digestive system using the example of enterocytes and hepatocytes. The positive effect of dry baker's yeasts, which is manifested in an increase in the absorption of enterocyte surface and an improvement in the locality in glycocalyx, both for enzymes and symbiotic microorganisms playing an active role in extracellular digestion, has been shown. In addition, no toxic effect of a high concentration of baker's yeasts in the diet on the organism of Russian sturgeon fry was registered. Scientific novelty. For the first time the analysis of the effect of different concentrations of inactivated baker's yeast, as part of the diet of Russian sturgeon fry, on the morphometric parameters of its digestive system has been performed. Practical value. Dry inactivated baker's yeasts were proved to be advisable to add to the starter feed for the Russian sturgeon as a biologically active dietary supplement because they have a positive effect on fish digestive system.