Lack of repeatability in red muscle mitochondrial function over time and across oxygen conditions in a teleost fish

Mitochondrial aerobic metabolism varies among individuals and has been linked to components of animal performance and fitness. However, because assessments of mitochondrial function have often required terminal sampling, most studies rely on single measurements, leaving the temporal and environmental consistency of mitochondrial traits within individuals largely unknown. Here, we investigated the repeatability of mitochondrial function measurements over time and across environmental conditions using repeated measurements in permeabilized red muscle of individual European sea bass (Dicentrarchus labrax). Mitochondrial metabolic traits were measured at monthly intervals under stable normoxic conditions and following exposure to hypoxia. At the population level, mean mitochondrial function was largely stable over time, with the exception of an increase in cytochrome c oxidase activity under hypoxia. However, mitochondrial respiratory rates, ATP production, and efficiency showed no repeatability within individuals over a one-month period across either stable or changing oxygen conditions. These results indicate that, although individuals differ markedly in mitochondrial function at any given time, mitochondrial phenotypes permeabilized red muscle are highly dynamic and context-dependent. Our findings highlight important limitations in the interpretation of single time-point measurements of mitochondrial metabolism and have implications for studies linking mitochondrial function to performance, ecological responses, and fitness.