Rates of hypoxia induction alter mechanisms of O2 uptake and the critical O2 tension of goldfish

The rate of hypoxia induction (RHI) is an important but overlooked dimension of environmental hypoxia that may affect an organism's survival. We hypothesized that, compared with rapid RHI, gradual RHI will afford an organism more time to alter plastic phenotypes associated with O-2 uptake and subsequently reduce the critical O-2 tension (P-crit) of the rate of O-2 uptake (M-O2). We investigated this by determining Pcrit values for goldfish exposed to short (similar to 24 min), typical (similar to 84 min) and long (similar to 480 min) duration P-crit trials to represent different RHIs. Consistent with our predictions, long duration P-crit trials yielded significantly lower P-crit values (1.0-1.4 kPa) than short and typical duration trials, which did not differ (2.6 +/- 0.3 and 2.5 +/- 0.2 kPa, respectively). Parallel experiments revealed these timerelated shifts in P(cri)t were associated with changes to aspects of the O-2 transport cascade that took place over the hypoxia exposures: gill surface areas and haemoglobin-O-2 binding affinities were significantly higher in fish exposed to gradual RHIs over 480 min than fish exposed to rapid RHIs over 60 min. Our results also revealed that the choice of respirometric technique (i. e. closed versus intermittent) does not affect Pcrit or routine M-O2, despite the significantly reduced water pH and elevated CO2 and ammonia levels measured following closed-circuit P-crit trials of similar to 90 min. Together, our results demonstrate that gradual RHIs result in alterations to physiological parameters that enhance O-2 uptake in hypoxic environments. An organism's innate P-crit is therefore most accurately determined using rapid RHIs (< 90 min) so as to avoid the confounding effects of hypoxic acclimation.