The fallacy of thePcrit – are there more useful alternatives?

Abstract

P critgenerally defined as the P O2 below which the animal can no longer maintain a stable rate of O 2 consumption (Ṁ O2), such that Ṁ O2 becomes dependent upon P O2provides a single number into which a vast amount of experimental effort has been invested. Here, with specific reference to water-breathers, I argue that this focus on the P crit is not useful for six reasons: (1) calculation of P crit usually involves selective data editing; (2) the value of P crit depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) P crit is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) P crit is not a reliable index of hypoxia tolerance; and (6) P crit carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full Ṁ O2 versus P O2 profiles accompanied by measurements of ventilation, lactate appearance and metabolic rate by calorimetry. If the goal is to assess the ability of the animal to regulate Ṁ O2 from this profile in a mathematical fashion, promising, more informative alternatives to P crit are the regulation index and Michaelis-Menten or sigmoidal allosteric analyses.

Standardizing the determination and interpretation ofPcritin fishes

“…This variability occurred even though P crit trials were conducted after > 24 h fasting, after 8-12 h since transferring fish to the respirometer, and during the dark phase of the photoperiod, when this species is less active. Owing to the variability of M O2 at high P O2 , the use of BSR is frequently coupled with removal of M O2 data points that fail to meet certain criteria (see Claireaux and Chabot, 2016 and Wood, 2018 for examples). This practice has raised concern over the rationale and validity of applying data selection criteria (Claireaux and Chabot, 2016; Wood, 2018).…”

“…Owing to the variability of M O2 at high P O2 , the use of BSR is frequently coupled with removal of M O2 data points that fail to meet certain criteria (see Claireaux and Chabot, 2016 and Wood, 2018 for examples). This practice has raised concern over the rationale and validity of applying data selection criteria (Claireaux and Chabot, 2016; Wood, 2018). In addition, direct comparisons of BSR with various non-linear regression approaches have shown that BSR is seldom the best model to fit M O2 data across a range of P O2 (Marshall et al, 2013; Cobbs and Alexander, 2018).…”

“…Perhaps the most common metric of the oxygen dependence of aerobic metabolism is the critical oxygen tension, P crit . For an animal that is capable of regulating its metabolism over a broad range of oxygen levels (an oxy-regulator), P crit represents the P O2 where oxygen consumption ( M O2 ) switches from being independent to being dependent on P O2 with further decreases in ambient oxygen (Ultsch et al, 1981; Rogers et al, 2016; Wood, 2018). Alternatively, P crit has been defined as the P O2 below which an animal’s basic metabolic needs, i.e.…”

“…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.…”

Interspecific differences and ecological correlations of energy metabolism traits in freshwater fishes

“…Previous studies of some fish species showed that the P crit was positively correlated with resting metabolism (Fu et al, ; Mandic et al, ) and, thus, suggested that a low resting metabolism potentially facilitates hypoxia tolerance (Killen et al, ). This might be because EPOC h is an inappropriate indicator of hypoxia tolerance (as P crit might also be; see details in the next paragraph, Wood, ), as we mentioned in the introduction. In this study, no significant relationship was observed between the RMR h and LOE at the interspecific level (Figure d).…”