The energetic consequences of oxygen fluxes in a coral reef fish

Abstract Ocean warming elevates metabolic rates in marine ectotherms but often constrains energetic resources, causing an imbalance between supply and demand. Transient hypoxia is near-ubiquitous across the world's coral reefs and may exacerbate this imbalance, yet its effects on the energetics of reef fishes remain poorly understood. In this study, we assess the metabolic costs incurred by a cryptobenthic reef fish exposed to oxygen fluxes measured on the world's hottest coral reefs in the Arabian/Persian Gulf. Hypoxia-exposure induced an 8.67% increase in aerobic metabolic rate over the six hours following reoxygenation, and resulted in an estimated 2.87% increase in total daily metabolic rate (mg O2 kg -1 day-1). This energetic cost did not coincide with detectable changes in anaerobic metabolism but was accompanied by increased activity during reoxygenation and a strong, acute transcriptomic response in genes related to oxygen-sensing. Oxygen availability on the reefs declined below the threshold for inducing such energetic costs on over half (56.04%) of the days throughout the summer, potentially leading to substantial cumulative costs. Such energetic costs represent an additional and previously under-appreciated consequence of hypoxia in coral reef environments that may exacerbate the temperature-induced mismatches between energy supply and demand, a key balance affecting growth and fitness. Overall design: 24 samples in total, split into 4 groups of 6 replicates. Each replicate is a cross section from the anterior tail of an Ecsenius pulcher. The sections were trimmed of skin prior to homogenisation. Fish are from either the 'hypoxic' group (N = 12), or the time-matched 'control' group (N = 12). Within each group, samples were taken either during hypoxia, or following reoxygenation and recovery from hypoxia. Hypoxia exposures followed the profiles measured on local reefs.