The Regulation Index: A New Method for Assessing the Relationship between Oxygen Consumption and Environmental Oxygen

Tropical coral reef organisms are predicted to be especially sensitive to ocean warming because many already live close to their upper thermal limit, and the expected rise in ocean CO 2 is proposed to further reduce thermal tolerance. Little, however, is known about the thermal sensitivity of a diverse and abundant group of reef animals, the gastropods. The humpbacked conch (Gibberulus gibberulus gibbosus), inhabiting subtidal zones of the Great Barrier Reef, was chosen as a model because vigorous jumping, causing increased oxygen uptake (Ṁ O2 ), can be induced by exposure to odour from a predatory cone snail (Conus marmoreus). We investigated the effect of present-day ambient (417-454 µatm) and projected-future (955-987 µatm) P CO2 on resting (Ṁ O2,rest ) and maximum (Ṁ O2,max ) Ṁ O2 , as well as Ṁ O2 during hypoxia and critical oxygen tension (P O2,crit ), in snails kept at present-day ambient (28°C) or projectedfuture temperature (33°C). Ṁ O2,rest and Ṁ O2,max were measured both at the acclimation temperature and during an acute 5°C increase. Jumping caused a 4-to 6-fold increase in Ṁ O2 , and Ṁ O2,max increased with temperature so that absolute aerobic scope was maintained even at 38°C, although factorial scope was reduced. The humpbacked conch has a high hypoxia tolerance with a P O2,crit of 2.5 kPa at 28°C and 3.5 kPa at 33°C. There was no effect of elevated CO 2 on respiratory performance at any temperature. Long-term temperature records and our field measurements suggest that habitat temperature rarely exceeds 32.6°C during the summer, indicating that these snails have aerobic capacity in excess of current and future needs.

Section: Discussion General Aspects Of Aerobic Performance Of the Hummentioning

confidence: 99%

Will jumping snails prevail? Influence of near-future CO2, temperature and hypoxia on respiratory performance in the tropical conchGibberulus gibberulus gibbosus

Lefevre

Watson

Munday

et al. 2015

“…There are different methods to calculate P crit , but analyses performed by Mueller and Seymour (2011) suggest that most of the methods used yield comparable values. We therefore decided to use a variation on a two-segment linear regression model (details below) to identify P crit as the Pw O2 at which the two linear trend lines (one representing the Pw O2 range of oxyregulation, the other of oxyconformation) intersect on a graph plotting ṀO 2 as a function of Pw O2 (BASIC program of Yeager and Ultsch, 1989).…”

Section: Co 2 and Nitrogenous End-product Measurementsmentioning

confidence: 99%

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

Regan

Richards

2017

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 (Ṁ O2 ). We investigated this by determining P crit values for goldfish exposed to short (∼24 min), typical (∼84 min) and long (∼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 crit 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 P crit or routine Ṁ O2 , despite the significantly reduced water pH and elevated CO 2 and ammonia levels measured following closed-circuit P crit trials of ∼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.

“…Oxygen is a fundamental requirement of most organisms, and its availability may be limiting across a range of habitats (Pörtner, 2010;Verberk et al, 2011;Ferguson et al, 2013). For over 30years, measures of oxygen tolerance have accumulated in a wide variety of taxa with variation evident at a range of scales (Greenlee and Harrison, 2004a;Mueller and Seymour, 2011;Lease et al, 2012;Ferguson et al, 2013). Surprisingly, no clear consensus over how to estimate oxygen tolerance exists, and for the most part, modern statistical approaches have not been brought to bear on this problem.…”

Section: Introductionmentioning

confidence: 99%

“…The broken stick approach to estimating P c has been applied in a range of contexts, and been used to demonstrate, for example, that the P c of a species is generally matched to the minimum oxygen level encountered in the environment in which it lives (Childress and Seibel, 1998;Nilsson, 2007;Ferguson et al, 2013), and that mobile species show behavioural avoidance of oxygen levels below their P c (Burleson et al, 2001). Nonetheless, not all species show clear break points in the relationship between the rate of oxygen consumption (V O2 ) and P O2 , which complicates efforts to assess the regulatory ability these species (Mueller and Seymour, 2011).…”

Section: Introductionmentioning

confidence: 99%

“…More generally, there are compelling reasons for describing continuous traits with functional relationships, rather than taking isolated point measures (Stinchcombe and Kirkpatrick, 2012). For example, Mueller and Seymour (Mueller and Seymour, 2011) fit a nonlinear function to estimate the ability of organisms to regulate oxygen consumption beyond simple oxy-conformity across a range of oxygen values: they propose a 'regulation index', which estimates a relative measure of oxyregulatory ability using either linear, quadratic (y=a+bx+cx 2 ) or one-phase association [y=y 0 +(y max -y 0 )(1-e -kx )] fits. Here, we apply a similar logic for estimating P c that meets the above criteria -a nonlinear regression approach coupled with simple differential calculus.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Estimating physiological tolerances - a comparison of traditional approaches to nonlinear regression techniques

Marshall

Bode

White

2013

SUMMARYTraditionally, physiologists have estimated the ability of organisms to withstand lower partial pressures of oxygen by estimating the partial pressure at which oxygen consumption begins to decrease (known as the critical P O2 or P c ). For almost 30years, the principal way in which P c has been estimated has been via piecewise ʻbroken stickʼ regression (BSR). BSR was a useful approach when more sophisticated analyses were less available, but BSR makes a number of unsupported assumptions about the underlying form of the relationship between the rate of oxygen consumption and oxygen availability. The BSR approach also distils a range of values into a single point with no estimate of error. In accordance with more general calls to fit functions to continuous data, we propose the use of nonlinear regression (NLR) to fit various curvilinear functions to oxygen consumption data in order to estimate P c . Importantly, our approach is back-compatible so that estimates using traditional methods in earlier studies can be compared with data estimates from our technique. When we compared the performance of our approach relative to the traditional BSR approach for real world and simulated data, we found that under realistic circumstances, NLR was more accurate and provided more powerful hypothesis tests. We recommend that future studies make use of NLR to estimate P c , and also suggest that this approach might be more appropriate for a range of physiological studies that use BSR currently.