Filling the gap in current knowledge of basic haematology of chimaeras – the case of wild‐caught American elephantfish <i>Callorhinchus callorynchus</i>

Haematological parameters have become important tools for monitoring natural changes as well as anthropogenic impacts on wild populations of fish. However, for holocephalans, we still have major gaps to fill regarding basic aspects of their haematology. We present the first detailed description of peripheral blood cells of the chimaera American elephantfish Callorhinchus callorynchus together with data on haematological parameters routinely used in animal health assessment. Adults of both sexes (n = 10) were captured in an area with low anthropogenic impact, a gulf in northeastern Patagonia, Argentina. Five major leukocyte types were identified and quantified: lymphocytes (63.33%), heterophils (18.82%), eosinophils (9.8%), neutrophils (4.47%) and monocytes (2.02%). No data on differential leukocyte counts were found for any other free-living holocephalan; however, comparison with values reported for the spotted ratfish Hydrolagus colliei in an aquarium environment showed marked differences. Immature erythroid precursors were regularly found in the peripheral blood of C. callorynchus, whereas no basophils or granulated thrombocytes were observed. Haematocrit values were higher in males than in females. The mean haematocrit for both sexes together was similar to values for its congener the ghost shark Callorhinchus milii and other chimaeras, resembling haematocrits of the least active elasmobranchs, but was relatively low compared to teleost fishes. Our study contributes to the comparative haematology of fishes by including novel information on a free-living holocephalan and starts a dataset of baseline haematological parameters for future studies assessing health status in the context of conservation physiology.