Circulatory oxygen transport in the water flea Daphnia magna

To determine the contribution of circulatory convection to tissue oxygen supply in animals of Daphnia magna, heart rate (f(H)), in-vivo Hb oxygen-saturation (S-Hb) and NADH fluorescence intensity (I-NADH) as a measure of the tissue oxygenation state were simultaneously measured using digital motion analysis, micro-absorption spectroscopy and fluorescence microscopy. In addition, the relationship between stroke volume and body size was established. Groups of differently sized animals (small: 1.4-1.6 mm, medium: 2.7-2.9 mm, large: 3.3 mm) with either low (Hb-poor) or high Hb concentration (Hb-rich) in the hemolymph were exposed to a gradual decrease in ambient oxygen partial pressure (P-O2amb) between normoxia and anoxia. In all groups, f(H) increased in response to progressive hypoxia. The hypoxic maximum in,f(H) was highest in medium-sized Hb-poor animals, whereas perfusion rate increased continuously with increasing body size in Hb-poor and Hb-rich animals. The P-O2amb at which Hb in the heart region was half-saturated (in-vivo P-50) was higher in medium-sized (Hb-poor: 3.2 kPa, Hb-rich: 2.0 kPa) than in small (Hb-poor: 2.1 kPa, Hb-rich: 1.5 kPa) and large animals (Hb-poor: 1.9 kPa). The in-vivo P-50 was always lower in Hb-rich than in Hb-poor animals. The I-NADH indicated an impairment of tissue oxygenation starting at higher critical P-O2amb with increasing body size and with lower Hb concentration. Model calculations suggest that at the respective critical P-O2amb, circulatory convection delivers less than half of the oxygen demand in Hb-poor animals. In contrast, in Hb-rich animals, the contribution of circulatory convection to tissue oxygen supply at respective critical P-O2amb was much greater due to the higher concentration of Hb.