AUTHOR=Frank LeeAnn C. , Prescott Leteisha A. , Scott Molly E. , Domenici Paolo , Johansen Jacob L. , Steffensen John Fleng 

TITLE=The effect of progressive hypoxia on swimming mode and oxygen consumption in the pile perch, Phanerodon vacca

JOURNAL=Frontiers in Fish Science

VOLUME=2

YEAR=2024

URL=https://www.frontiersin.org/journals/fish-science/articles/10.3389/frish.2024.1289848

DOI=10.3389/frish.2024.1289848

ISSN=2813-9097

ABSTRACT=<sec><title>Introduction</title><p>Hypoxia, an increasingly common stressor in coastal environments, lowers the scope for aerobic activity such as sustained swimming. This study examines the effect of self-depleting progressive hypoxia on swimming performance and oxygen consumption of the pile perch, <italic>Phanerodon vacca</italic>, at their optimal speed (U<sub>opt</sub> =29 cm·s<sup>−1</sup>). <italic>P. vacca</italic> is a labriform, median-paired fin (MPF) swimmer that exhibits a clear gait transition from primarily oxidative muscle-powered, pectoral fin swimming to primarily anaerobic-powered muscle burst swimming using the caudal fin (BCF) when facing high speeds or low oxygen.</p></sec><sec><title>Methods and hypothesis</title><p>We expected that <italic>P. vacca</italic> swimming at U<sub>opt</sub> would maintain oxygen consumption (<italic>ṀO</italic><sub>2</sub>) alongside decreasing oxygen levels and continue to swim using MPF propulsion until they approached their critical oxygen saturation at their optimal swimming speed (S<sub>crit</sub> at U<sub>opt</sub>). At this point, we expected a gait transition to occur (i.e., from MPF to BCF propulsion), which is observed by a decrease in pectoral fin beat frequency and an increase in caudal fin or bursting frequency.</p></sec><sec><title>Results</title><p>In a closed-system swimming respirometer, <italic>P. vacca</italic> maintained strictly pectoral fin swimming at a consistent frequency and metabolic rate until reaching a critical oxygen saturation at their S<sub>crit</sub> at U<sub>opt</sub> of 38.6 ± 1.7% air saturation (O<sub>2sat</sub>). Below S<sub>crit</sub> at U<sub>opt</sub>, <italic>P. vacca</italic> significantly increased pectoral fin beat frequency, followed by a transition to caudal bursting at 33.7% O<sub>2sat</sub>. Switching to burst swimming allowed <italic>P. vacca</italic> to swim for 44.4 min beyond S<sub>crit</sub> at U<sub>opt</sub> until reaching 29.2% O<sub>2sat</sub>. Excess post-hypoxia oxygen consumption (EPHOC) led to a significant increase in metabolic rate during recovery, which took 1.89 h to return to the routine metabolic rate (RMR).</p></sec><sec><title>Discussion</title><p>Time to return to RMR and EPHOC did not differ when comparing exhaustive exercise and hypoxia exposure, suggesting that this species has an anaerobic energy reserve that does not differ when stressed during hypoxia or exercise. This study demonstrates that in hypoxia, the modulation of swimming mode from pectoral to caudal fin–based locomotion can maintain swimming well below S<sub>crit</sub> at U<sub>opt</sub> and provides a fundamental understanding of the physiological basis of sustained swimming in hypoxia.</p></sec>