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 (P_O2amb). 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 P_O2amb values, marking the significant decrease in appendage beating rate, the intersection of two solid lines or the P_O2amb at which 50 % of the Hb was saturated. The only exception concerns the critical P_O2amb 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 P_O2amb 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 P_O2amb at which the animals were reared.