Fig. 6. Responses of appendage beating rate, NADH fluorescence intensity, heart rate and haemoglobin (Hb) oxygen-saturation of normoxia-acclimated Hb-poor (left) and hypoxia-acclimated Hb-rich Daphnia magna (right) to decreasing ambient oxygen partial pressures (PO2amb). Each data point represents the mean ± S.D. (N=6). Solid lines represent either plateaus in the physiological data or mean regression lines obtained by averaging the variables of individual regression lines. Dashed lines and the respective values represent critical PO2amb values, marking the significant decrease in appendage beating rate, the intersection of two solid lines or the PO2amb at which 50 % of the Hb was saturated. The only exception concerns the critical PO2amb in C (see Fig. 7). NADH fluorescence intensity was trend-corrected (see Fig. 4E) and normalized to 100 %. Sigmoidal curves were generated by nonlinear regression analysis using the Hill equation, which yielded the PO2amb at which 50 % of Hb was saturated, and the Hill coefficient n as a measure of sigmoidity (r2>0.990 for both curves). The two arrows on the abscissa indicate the PO2amb at which the animals were reared.
