Blood cells and serum chemistry in the world’s largest fish: the whale shark Rhincodon typus

Descriptive information regarding the peripheral blood of normal whale sharks Rhincodon typus Smith 1828 is presented based on samples collected from 2 healthy female specimens held in an aquarium collection. Erythrocyte morphology was similar to other orectolobiforms and major leukocyte types were similar to other non-carcharhinid sharks. The numerically dominant population was the lymphocyte (46%), followed by the fine eosinophilic granulocyte or heterophil (39.5%). The remaining 15% of white blood cells were divided among coarse eosinophilic granulocytes or eosinophils, simple neutrophils, monocytes similar to those of most elasmobranchs and a rare basophilic stippled granulocyte or basophil. Thrombocytes were similar to those of other shark species; no granulated thrombocytes were observed in this species. Blood gas data and serum chemistry values are also presented. For comparison, 22 blood samples collected from 2 moribund male specimens showed that the heterophil/lymphocyte ratio may be a potentially useful biomarker of whale shark health. A pattern of marked heterophilia in these animals became more pronounced as their clinical condition deteriorated. These data represent the first exploration of the internal biology of the world’s largest fish.