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This study asked whether interindividual variation in maximum and standard aerobic metabolic rates of the Gulf killifish, Fundulus grandis, correlates with gill morphology and cardiac mitochondrial bioenergetics, traits reflecting critical steps in the O2 transport cascade from the environment to the tissues. Maximum metabolic rate (MMR) was positively related to body mass, total gill filament length and myocardial oxygen consumption during maximum oxidative phosphorylation (multiple R2=0.836). Standard metabolic rate (SMR) was positively related to body mass, total gill filament length and myocardial oxygen consumption during maximum electron transport system activity (multiple R2=0.717). After controlling for body mass, individuals with longer gill filaments, summed over all gill arches, or greater cardiac respiratory capacity had higher whole-animal metabolic rates. The overall model fit and the explanatory power of individual predictor variables were better for MMR than for SMR, suggesting that gill morphology and myocardial bioenergetics are more important in determining active rather than resting metabolism. After accounting for body mass, heart ventricle mass was not related to variation in MMR or SMR, indicating that the quality of the heart (i.e. the capacity for mitochondrial metabolism) was more influential than heart size. Finally, the myocardial oxygen consumption required to offset the dissipation of the transmembrane proton gradient in the absence of ATP synthesis was not correlated with either MMR or SMR. The results support the idea that interindividual variation in aerobic metabolism, particularly MMR, is associated with variation in specific steps in the O2 transport cascade.

Standard metabolic rate (SMR), maximum metabolic rate (MMR), absolute aerobic scope (AAS), and critical oxygen tension (Pcrit) were determined for the Gulf killifish,Fundulus grandis, an ecologically dominant estuarine fish, acclimated to lowered salinity, elevated temperature, and lowered oxygen concentration. Acclimation to low salinity resulted in a small, but significant, elevation of Pcrit; acclimation to elevated temperature increased SMR, MMR, AAS, and Pcrit; acclimation to low oxygen led to a small increase in SMR, but substantial decreases in MMR, AAS, and Pcrit. Variation in these metabolic traits among individuals was consistent and repeatable when measured during multiple control exposures over seven months. Trait repeatability was unaffected by acclimation condition suggesting that repeatability of these traits is not context dependent. There were significant phenotypic correlations between specific metabolic traits: SMR was positively correlated with MMR and Pcrit; MMR was positively correlated with AAS; and AAS was negatively correlated with Pcrit. In general, within-individual variation contributed more than among-individual variation to these phenotypic correlations. The effects of acclimation on these traits demonstrate that aerobic metabolism is plastic and influenced by the conditions experienced by these fish in the dynamic habitats in which they occur; however, the repeatability of these traits and the correlations among them suggest that these traits change in ways that maintains the rank order of performance among individuals across a range of environmental variation.SUMMARY STATEMENTAerobic metabolism of an ecologically dominant estuarine fish is influenced by acclimation to environmental changes without altering trait repeatability. Furthermore, specific metabolic traits are phenotypically correlated.

For most fishes, there is an oxygen level, the critical oxygen tension (Pcrit), below which oxygen consumption (MO2) becomes dependent upon ambient oxygen partial pressure (PO2). We compare multiple curve-fitting approaches to estimatePcritof the Gulf killifish,Fundulus grandis, during closed and intermittent-flow respirometry. The traditional approach fitting two line segments ofMO2versusPO2produced high and variable estimates ofPcrit. Nonlinear regression using hyperbolic or Weibull functions resulted in either variablePcritestimates or, in some cases, failed to converge upon meaningful solutions.Pcritdetermined as thePO2whenMO2equals standard metabolic rate (SMR) based upon a linear relationship ofMO2andPO2at lowPO2were consistent across fish and experimental trials. Therefore, we recommend that Pcrit specifically refer to thePO2below which SMR cannot be maintained. Its determination, therefore, requires accurate measurement of SMR.