Size-invariant exercise-induced aerobic scope across a broad body mass range in a giant isopod

Aerobic scope is the aerobic metabolism left after maintenance costs are met, and it sets a simple limit on how much oxygen-dependent activity an animal can support. In many aquatic animals, this margin changes with body size because resting and active metabolic rates do not always scale in the same way. Large deep-sea scavengers are interesting in this respect because they live where food is sparse and unpredictable, but they must still move when carrion or food cues appear. We measured resting metabolic rate (RMR) and post-exercise peak oxygen consumption, here referred to as active metabolic rate (AMR), in the giant isopod Bathynomus doederleini across a body-mass range of 1.7-48.4 g using intermittent-flow respirometry at 10;. RMR and AMR both increased with body mass and had similar scaling exponents. As a result, factorial aerobic scope (FAS = AMR/RMR) did not change with body mass (median FAS = 2.83). A residual analysis also showed that mass-corrected RMR was not significantly related to mass-corrected AMR. Thus, individuals with relatively high maintenance metabolism did not necessarily have higher post-exercise aerobic capacity. These results show that exercise-induced aerobic scope was maintained across ontogeny in this deep-sea scavenger. They also show that body size and individual metabolic phenotype provide different information: body size explained the shared increase in RMR and AMR, whereas residual variation did not show a strong link between the two traits.