Metabolic, respiratory and cardiovascular responses to acute and chronic hypoxic exposure in tadpole shrimp Triops longicaudatus

Hypoxic exposure experienced during sensitive developmental periods can shape adult physiological capabilities and define regulatory limits. Tadpole shrimp were reared under normoxic (19-21 kPa O₂), moderate (10-13 kPa O₂) or severe (1-3 kPa O₂) hypoxic conditions to investigate the influence of developmental oxygen partial pressure (P _O2) on adult metabolic, respiratory and cardiovascular physiology. Developmental P_O2 had no effect on metabolic rate or metabolic response to hypoxic exposure in adults. All rearing groups decreased O ₂ consumption as water P_O2 decreased. Heart rate, stroke volume and cardiac output were independent of P_O2 down to 5 kPa O₂ in all rearing groups. Below this, cardiac output was maintained only in tadpole shrimp reared under severe hypoxic conditions. The enhanced ability to maintain cardiac output was attributed to an increase in hemoglobin concentration and O₂-binding affinity in those animals. Oxygen-delivery potential was also significantly higher in the group reared under severe hypoxic conditions (1336 μl O₂ min^-1) when compared with the group reared under normoxic conditions (274 μl O ₂ min^-1). Differences among the rearing groups that were dependent on hemoglobin were not considered developmental effects because hemoglobin concentration could be increased within seven days of hypoxic exposure independent of developmental P_O2. Hypoxia-induced hemoglobin synthesis may be a compensatory mechanism that allows tadpole shrimp to regulate O₂ uptake and transport in euryoxic (O₂ variable) environments. The results of this study indicate that increased hemoglobin concentration, increased O₂-binding affinity and transient decreases in metabolic demand may account for tadpole shrimp hypoxic tolerance.