Rates of hypoxia induction alter mechanisms of O uptake and the critical O 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 uptake and subsequently reduce the critical O tension () of the rate of O uptake ( ). We investigated this by determining values for goldfish exposed to short (∼24 min), typical (∼84 min) and long (∼480 min) duration trials to represent different RHIs. Consistent with our predictions, long duration trials yielded significantly lower 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 time-related shifts in were associated with changes to aspects of the O transport cascade that took place over the hypoxia exposures: gill surface areas and haemoglobin-O 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 or routine , despite the significantly reduced water pH and elevated CO and ammonia levels measured following closed-circuit trials of ∼90 min. Together, our results demonstrate that gradual RHIs result in alterations to physiological parameters that enhance O uptake in hypoxic environments. An organism's innate is therefore most accurately determined using rapid RHIs (<90 min) so as to avoid the confounding effects of hypoxic acclimation.