Hemoglobin-oxygen affinity and body weight in the American plaice, Hippoglossoides platessoides

Hemoglobin-oxygen affinity (P₅₀) as indicated by blood oxygen dissociation curves, was measured in the American plaice, Hippoglossoides platessoides. Body weight (W), hemoglobin concentration (Hb) and intraerythrocytic nucleoside triphosphate concentration (NTP) were measured. A negative correlation between W and P₅₀ indicated that smaller fish have hemoglobin with a decreased affinity for oxygen, compared to large fish. The negative correlation between NTP/Hb and W provided an explanation as to how the smaller fish maintain a decreased hemoglobin-oxygen affinity. Smith (1977) has proposed that small fish have a decreased blood oxygen solubility (αBo₂). He suggested that smaller fish maintain their high metabolic rate per unit weight (Vo₂/W), despite a decreased αBo₂ by maintaining a larger arteriovenous oxygen partial pressure difference (Pa,o₂ - Py,o₂) 1 than do large fish. This hypothesis is strongly supported by evidence presented here.