Illustrations of how P_crit and metabolic reduction capacity were calculated.

A, B and C: Examples of changes in

M˙

O2

as a function of oxygen tension for individual M. fluviatilis, Hypseleotris and M. adspersa (respectively), demonstrating the abrupt decline in

M˙

O2

that defines P_crit. On each plot three lines are presented. The red line is the regression defining the oxyconformation zone of gradual hypoxia, while the orange line is the regression defining the oxyregulation zone during gradual hypoxia. Both the red and orange lines were determined using the algorithm of Yeager and Ultsch, and their point of intersection is one way of calculating P_crit (P_crit,YU). An alternative estimator of P_crit (P_crit,SMR) is the point of intersection of SMR (blue line) and the regression defining oxyconformation (red line). D. Reduction Capacity (RC_SMR in this case), was calculated as the logarithm of the ratio of two areas (ln(A_r/A_e)), where A_r is the area between the SMR and depressed metabolic rate curves, during gradual hypoxia, and A_e is excess post-hypoxic oxygen consumption (EPHOC). Two points of intersection between the SMR curve and the fitted spline curve, and two points of intersection between the RMR curve and the spline, define four times critical for determination of the integrals defining A_r and A_e: t_startR, t_finishRt_startE and t_finishE are, respectively, the times at which (a) metabolic reduction below SMR began; (b) metabolic reduction ceases; (c) EPHOC begins; (d) EPHOC finishes (see Materials and Methods). (Data presented in D is from a 4.67 g M. fluviatilis).