During ichthyological research within the framework of the Joint Russian-Mongolian Complex Biological Expedition of the Russian Academy of Sciences and the Academy of Sciences of Mongolia, ichthyologists were quite surprised by some strange habits of Mongolian fish, which is of course connected with the specific conditions of the reservoirs they inhabit in the semi-arid zone of the highlands with a sharply continental climate.
In the reservoirs of the western part of the Central Asian endorheic basin of Mongolia, against the background of harsh climatic conditions, lakes and rivers have low biological productivity and low species diversity of fish. At the same time, one species, Potanin's Altai osman Oreoleuciscus potanini (Kessler, 1879) is an absolute dominant in terms of numbers and biomass. This species has four intraspecific forms: piscivorous, herbivorous, sharp-snouted and riverine, differing in morphology and ecology. What the ichthyologists found strange was not this way of solving the problem of biodiversity (the "bundles of forms" in fish are observed in many reservoirs of the globe with specific conditions and depleted ichthyofauna), but what the Altai osmans feed on. It turned out that the intestinal contents of the most numerous herbivorous form mainly (up to 80%) consists of charophytes. Considering that Chara thalli are encrusted with calcium carbonate, which makes up to 70% of the dry mass, it was unclear what organic matter O. potanini could assimilate from such food.
To solve this riddle, studies were conducted in the Durgun Reservoir (48° 19′ 33″ N, 92° 48′ 25″ E) of the Great Lakes Basin, located in the western part of the Central Asian endorheic basin (Fig. 1, 2, 3). According to net catches, the share of the herbivorous form in the reservoir was 75.1% of all forms of O. potanini (Fig. 4). Samples of fish muscle tissue were taken - O. potanini, a branch of Chara spp. from the reservoir (Fig. 5), pre-cleaned from phytoperiphyton (periphytic microalgae) and the phytoperiphyton itself, washed off the hara.
"To find out what exactly Potanin's Altai osman gets from consuming charophytes, we performed stable isotope analysis (SIA) and a more detailed analysis of biomarkers - fatty acids (FA). SIA showed that O. potanini received its main carbon from periphytic microalgae, and not from Chara spp. The FA analysis generally confirmed the SIA results. In particular, biomarker FA of diatoms were found in the biomass of O. potanini, while characteristic biomarkers of Chara spp. were absent," said Doctor of Biological Sciences, Academician of the Russian Academy of Sciences, Head of the Laboratory of Ecology of Aquatic Communities and Invasions Yu. Yu. Dgebuadze.
Thus, the herbivorous form of O. potanini absorbed algae to assimilate periphytic microalgae during passage through the intestine. At the same time, the thalli of charophyte algae were not crushed by fish and left their bodies practically unchanged.
The work was carried out by scientists from the A. N. Severtsov Institute of Ecology and Evolution Problems of the Russian Academy of Sciences, Moscow, the Institute of Biophysics of the Federal Research Center "Krasnoyarsk Scientific Center" of the Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk, the Siberian Federal University, Krasnoyarsk, the Western Regional Branch of the National University of Mongolia, Khovd, Mongolia and the Institute of Geography and Geoecology of the Mongolian Academy of Sciences, Ulaanbaatar, Mongolia. The head of the Joint Russian-Mongolian Complex Biological Expedition of the Russian Academy of Sciences and the Academy of Sciences of Mongolia S. N. Bazha provided great assistance in conducting these studies.
The article is published in open access: Dgebuadze, Y.Y., Sushchik, N.N., Altansukh, D. Mendsaikhan, B., Emelianova, A.Yu., Gladyshev, M.I. Ingested vs. digested: what do Potanin’s Altai Osmans (Cypriniformes, Leuciscidae) really eat? Environ Biol Fish (2025).