Fig. 1. The effects of stepwise increases in water P_CO2 (Pw_CO2) followed by the rapid lowering of Pw_CO2 (A) on ventilation, blood gases, and acid-base status in tambaqui Colossoma macropomum, including (B) ventilation amplitude (V_AMP; N=9-11), (C) ventilation frequency (fV; N=8-9), (D) arterial P_CO2 (Pa_CO2; N=8-9), (E) arterial P_O2 (Pa_O2; N=7-8) and (F) arterial pH (pHa; N=8-9). Pre1 and Pre2 refer to pre-exposure values compiled 2 min and immediately, respectively, before the onset of hypercarbia; the numbers 7, 16 and 26 denote values compiled for the final 2 min of the three discrete steps of increasing hypercarbia; and the data plotted to the right of the break in the x-axis represent values compiled for 2 min intervals after the initiation of rapid washout of CO₂ at 0 min. The data are presented as means ± 1 S.E.M. For stepwise increases in Pw_CO2, values that do not share a letter are significantly different from one another (one-way RM-ANOVA, P<0.001 for B-D and F; P=0.001 for E). For the rapid washout of water CO₂, asterisks indicate values that are significantly different from the value at time=0 min (one-way RM-ANOVA, P<0.001 for all).