Abstract

The adaptive nature of haemoglobin function in a diverse range of aquatic ectothermic vertebrates is demonstrated by its intrinsic oxygen-binding properties and by erythrocyte cofactor modulation of Hb function. The selective advantage of heterogeneous isohaemoglobins and polymorphic expression of functionally distinct components is considered in relation to environmental oxygen tensions and temperature. The difficulty of comparing physiological adaptations in divergent species is emphasized. Recent population studies suggest that relatively minor differences in environmental conditions, particularly temperature, direct the expression of functionally heterogeneous haemoglobins, although the thresholds for expression have not been established. Regulatory mechanisms underpinning the molecular mechanisms for hypoxic induction of Hb in aquatic vertebrates are not well understood.