Benefits of haemoglobin in the cladoceran crustacean Daphnia magna

To determine the contribution of haemoglobin (Hb) to the hypoxia-tolerance of Daphnia magna, we exposed Hb-poor and Hb-rich individuals (2.4-2.8mm lon g) to a stepwise decrease in ambient oxygen partial pressure (PO2amb) over a period of 51min from normoxia (20.56kPa) to anoxia (<0.27kPa) and looked for differences in their physiological performance. The haem-based concentr ations of Hb in the haemolymph were 49 mu moll(-1) in Hb-poor and 337 mu mo ll(-1) in Hb-rich animals, respectively. The experimental apparatus made si multaneous measurement of appendage beating rate (fA), NADH fluorescence in tensity (I-NADH) of the appendage muscles, heart rate (fh) and in vivo Hb o xygen-saturation possible. In response to progressive, moderate hypoxia, bo th groups showed pronounced tachycardia and a slight decrease in fA. The fA and fH of Hb-rich animals were generally 4-6 % lower than those of Hb-poor animals. In addition, Hb-rich animals showed a significant decrease in the P-O2amb at which the Hb in the heart region was half-saturated and a strik ing reduction in the critical P-O2amb Of appendage-related variables. In Hb -poor animals, the INADH signal indicated that the oxygen supply to the lim b muscle tissue started to become impeded at a critical P-O2amb of 4.75 kPa , although the high level of fA was largely maintained until 1.77 kPa. The obvious discrepancy between these two critical P-O2amb values suggested an anaerobic supplementation of energy provision in the range 4.75-1.77 kPa. T he fact that I-NADH of Hb-rich animals did not rise until PO,amb fell below 1.32 kPa strongly suggests that the extra Hb available to Hb-rich animals ensured an adequate oxygen supply to the limb muscle tissue in the P-O2amb range 4.75-1.32 kPa. This finding illustrates the physiological benefit of Hb in enabling the animal to sustain its aerobic metabolism as the energeti cally most efficient mode of fuel utilization under conditions of reduced o xygen availability.