Blood cell morphology, ultrastructure and cytochemistry in marine mammals: a narrative comparative review

Marine mammals exhibit a range of hematological features associated with diving, hypoxia, and life in a high-pressure aquatic environment. This narrative review synthesizes available morphological, ultrastructural, and cytochemical data on blood cells in cetaceans, pinnipeds, and sirenians, with the aim of identifying common patterns and sources of interspecific variability. Erythrocytes in marine mammals are often relatively large and display species-specific differences in shape and indices that may contribute to differences in oxygen transport physiology and blood rheology. Leukocyte morphology and cytochemical characteristics vary considerably among species, potentially reflecting phylogenetic background, ecological influences, and other biological or methodological factors. Differences in granulocyte structure and cytochemical staining characteristics may indicate variation in immune cell phenotype across lineages, although their functional significance remains incompletely understood. Comparative analysis indicates that cytochemical staining patterns are not consistent across species, highlighting the need for species-specific reference data and caution in extrapolating from terrestrial mammals. Interpretation of available data is limited by uneven taxonomic coverage, small sample sizes, and methodological heterogeneity among studies. Key knowledge gaps include the functional implications of cytochemical diversity, the molecular mechanisms underlying hematological variation, and the lack of data for many taxa, particularly mysticetes. By integrating evidence across multiple levels of organization, this review provides a critical overview of current knowledge and highlights current limitations and priorities for future research in comparative physiology, veterinary medicine, and conservation biology.