Tadpole shrimp (Triops longicaudatus) responses to acute and developmental hypoxic exposure

Tadpole shrimp Triops longicaudatus (Crustacea: Branchiopoda: Notostraca) were reared under normoxic (19--21 kPa O2), moderate (10--13 kPa O2) or severe (1--3 kPa O2) hypoxic conditions to investigate the effects of developmental Po2 (partial pressure of oxygen) on adult metabolic, respiratory and cardiovascular physiology. Growth rates were independent of developmental Po2, although adult body size was significantly smaller in tadpole shrimp reared under severe hypoxic conditions relative to those reared under normoxic conditions. Most organisms increase respiratory surface area in response to hypoxic exposure. Yet growth of the epipodites (proposed respiratory structure) relative to mass (Y = aMb) was significantly reduced in tadpole shrimp reared under severe hypoxic (b = 1.17) conditions relative to those reared under normoxic (b = 1.78) or moderate hypoxic (b = 1.56) conditions. Despite the reduction in respiratory surface area, O2 consumption rates and metabolic response to hypoxic exposure did not differ among rearing groups. All groups reduced O2 consumption in response to hypoxic exposure. The primary function of the epipodites may not be respiratory gas exchange given that development of the epipodites was limited by developmental hypoxic exposure and the reduction did not alter O2 consumption rates. Alternatively, hypoxic exposure throughout development may promote the allocation of limited resources (O2) toward hemoglobin synthesis instead of respiratory surface anabolism.;Hypoxia-induced hemoglobin synthesis appears to be a compensatory mechanism that tadpole shrimp utilize to regulate O2 uptake and transport in euryoxic environments. Although hemoglobin concentrations differed among rearing groups, they were not dependent on developmental Po2 because hemoglobin synthesis could be induced by hypoxic exposure in adults. Adult tadpole shrimp reared under normoxic conditions increased hemoglobin concentration within seven days of exposure to severe hypoxic conditions. Within ten days, hemoglobin concentration increased to the levels observed in tadpole shrimp reared under severe hypoxic conditions. Tadpole shrimp reared under severe hypoxic conditions had an enhanced ability to maintain cardiac parameters when exposed to 2 kPa O2 due to their higher hemoglobin concentration and O2-binding affinity. 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.