Don't throw the fish out with the respirometry water

Abstract

Dr Wood's Commentary (Wood, 2018) provides six reasons to question the usefulness of P crit and proposes alternative ṀO 2 versus P O2 analyses as its replacement. While we agree with some of Dr Wood's arguments, we feel that none of them warrant abandoning P crit , especially in favour of his proposed alternatives, which provide different information than P crit . A more useful way forward would involve (1) clearly defining P crit , to avoid misinterpretation, and (2) standardizing (or at least clearly describing) the methods used to determine and report P crit , to optimize its comparative value. This topic demands further discussion because Dr Wood's conclusion could have unwarranted influence on how future hypoxia research is conducted and past hypoxia research is interpreted.Dr Wood's arguments are either theoretical (reasons 3-6) or methodological (reasons 1, 2). The theoretical arguments, if true, may warrant the abandonment of P crit . However, contrary to Dr Wood's claims, across species, P crit is strongly correlated with the environmental O 2 level to which species are exposed and is therefore ecologically relevant (Childress and Seibel, 1998;Mandic et al., 2009;Wishner et al., 2018). P crit is also correlated with multiple steps of the O 2 transport cascade, from gill surface area through haemoglobin P 50 to mitochondrial P 50 (Childress and Seibel, 1998;Lau et al., 2017;Mandic et al., 2009), and is a sensitive measure of an animal's overall ability to extract O 2 because altering physiological traits along the cascade can change P crit . For example, anatomical restructuring of the gill to favour O 2 diffusion [e.g. reducing gill epithelial thickness through seawater acclimation in sculpins (Henriksson et al., 2008); increasing lamellar surface area through hypoxia acclimation in crucian carp (Sollid et al., 2003)] can lower P crit . These relationships between P crit and plastic traits along the O 2 transport cascade clearly reflect the physiological relevance of P crit and indicate that P critespecially when it shifts with acclimationdoes indeed represent the P O2 at which O 2 uptake becomes constrained. Therefore, contrary to reason 6, P crit per se does carry biologically relevant information. Hence, P crit remains a useful tool for understanding hypoxic performance because it allows for predictive statements.Much of Dr Wood's reasoning centres on the over-interpretation of P crit . P crit does not necessarily quantify an animal's overall hypoxia tolerance (the product of some combination of aerobic metabolism, anaerobic metabolism and metabolic depression; reason 5), reveal what biological processes the O 2 consumed at P crit is supporting (reason 4), or indicate the onset P O2 of enhanced glycolytic reliance (reason 4) or metabolic depression (reason 5).These ideas have long been excluded from the definition of P crit . Simply, P crit defines the lowest water P O2 at which the animal can maintain some benchmark ṀO 2 state (e.g. ṀO 2,std , the ṀO 2 of an inactive and post-absorptive ectotherm...

Standardizing the determination and interpretation ofPcritin fishes

“…2C). Also, it is not clear that this parameter has any particular meaning when applied to whole animal M O2 , unlike its meaning in enzyme kinetics (Regan et al, 2019). In addition, the use of the parameter b assumes that the model fits the data well and that the upper asymptote of the MM function represents a definite, physiological maximum, neither of which were true in this study.…”

“…Recently, Wood (2018) questioned the usefulness of the P crit concept based on two main concerns: uncertainty of its biological meaning and lack of standardization in its determination. The purpose of this study is not to argue the biological relevance of P crit , as this concern has been addressed (Regan et al, 2019): rather, the purpose of this study is to evaluate analytical methods used to determine P crit from respirometric data. Traditionally, P crit has been estimated the intersection of two straight lines, one fit to a region where M O2 is relatively independent of P O2 and a second line describing the decrease in M O2 at low P O2 (Yeager and Ultsch 1989; Rogers et al, 2016).…”

Effects of hypoxia on aerobic metabolism and active electrosensory acquisition in the African weakly electric fish Marcusenius victoriae

“…Marcusenius victoriae 's p crit‐EOD (15.14 mmHg) is not significantly different from its p crit‐MR and further supports the hypothesis that this population of fish consists of swamp specialists. In addition, the concurrence of the p crit‐MR and the p crit‐EOD , the critical oxygen tension of a biologically relevant aerobic rate process, supports the use of p crit‐MR as an informative metric of hypoxia tolerance (Regan et al ., ; Wood, ).…”

“…This point is commonly termed p crit to denote the critical partial pressure of oxygen ( p O 2 ). p crit is often used as a metric for hypoxia tolerance and correlates well with other hypoxia tolerance measures, such as aquatic surface respiration thresholds and haemoglobin concentrations (Chapman et al ., ; Chapman & Mckenzie, ; Fish, ; Regan et al ., ). Because critical thresholds may exist for other oxygen‐dependent rate processes, the metabolic rate (MR) critical threshold is herein referred to as p crit‐MR .…”

Breathing new life into the critical oxygen partial pressure (Pcrit): a new definition, interpretation and method of determination

“…The ratio of environmental PO2 to Pcrit, normalized for temperature, provides a direct measure of aerobic scope that is believed to constrain the viability of current and future habitat (Deutsch et al, 2015;Deutsch et al, 2020). However, hypoxia and tolerance are subjective, time-dependent terms while Pcrit is inconsistently defined and determined, resulting in debate about its meaning and significance (Wood, 2018;Regan et al, 2019).…”