Morphological adaptation of erythrocytes of Cyprinids to the conditions of the Samara River (Dnipropetrovsk region)

Purpose. To investigate the pathomorphological and cytometric parameters of red blood cells in commercial fish species of the Cyprinidae family inhabiting the Samara River. Methodology. Fish were sampled in August-September 2023 in the mouth section of the Samara River. The study objects were red blood cells of three cyprinids: common bream (Abramis brama), common carp (Cyprinus carpio), and roach (Rutilus rutilus). For hematological studies, 10 age-3+ individuals of each fish species were collected. Blood was collected from the heart. Blood smears were stained using the Romanowsky-Giemsa method. In each smear, 100 fields of view were examined at 40x magnification. Morphometric examinations of erythrocytes were conducted using a universal computer analysis of microphotographs taken with a “Sciencelab T500 5.17 M” digital camera attached to a “Biolam 70” microscope. To characterize the state of red blood cells, a count of mature and immature blast forms of erythrocytes was performed; cytometric parameters of mature erythrocytes were determined, including measurements of the large longitudinal (D) and minor transverse (d) diameters, erythrocyte area (S), and erythrocyte nucleus area (s); ellipticity coefficients of erythrocytes, nucleus-cytoplasmic ratios, and the relative number of pathological changes in cells and nuclei were calculated. The obtained results were compared using statistical analysis with the Student's t-test. Findings. Mature erythrocytes constituted the largest group of red blood cells in all studied fish species (97.3–98.9%). The relative number of immature erythrocyte forms in carp was 1.5 times higher compared to bream and 2.5 times higher compared to roach. The longitudinal and transverse diameters of erythrocytes were significantly higher in carp and bream compared to roach. The ellipticity coefficient of erythrocytes in bream and carp had similar values (0.31±0.06 and 0.34±0.03, respectively) and was significantly higher than in roach. The area of mature erythrocytes was also the largest in carp and bream (78.33±1.012 μm² and 73.70±1.303 μm², respectively) and significantly exceeded that in roach (56.75±1.134 μm²). The highest nucleus-cytoplasmic ratio was observed in roach erythrocytes (0.45±0.004), exceeding this parameter in bream by 69% and in carp by 38%. Pathological changes in erythrocytes were typical for all three fish species; in carp, these changes mainly manifested as membrane deformation and cell shape changes (poikilocytosis). In bream and especially in roach, erythrocyte agglutination was more frequently observed. Nuclear pathologies (lysis, pyknosis) were mainly typical for bream and roach. Originality. This is the first study of the morphological and cytometric parameters of red blood cells in bream, carp, and roach inhabiting the Samara River; specific structural conditions of erythrocytes in different cyprinid species under elevated water mineralization, as a primary hydroecological factor of technogenic origin, have been identified. Practicalvalue. The study results can be used in fish farming practices to assess the adaptive capabilities of commercial fish species in response to high mineralization and other anthropogenic hydrochemical factors characteristic of the Samara River.