Standard and maximum metabolic rates (SMR, MMR) vary among individuals within a species in a reproducible fashion. Many morphological and physiological traits likely contribute to this variation, including the capacity for gas exchange at the gills and the ability of the heart to distribute blood to the tissues. Here, we estimated SMR and MMR by intermittent-flow respirometry of the Gulf killifish, Fundulus grandis, and determined whether these rates are correlated with gill morphology and cardiac bioenergetics. Gill filament length and filament number were determined as proxies of gill surface area. Oxygen consumption by permeabilized heart ventricles was measured by high-resolution respirometry at saturating substrate concentrations during the following respiratory states; LEAK, OXPHOS (ADP-stimulated respiration), and ET (maximum noncoupled respiration). General linear modeling showed that SMR was best predicted by body mass, total filament length, and cardiac oxygen consumption during ET (r² = 0.69), while MMR was best predicted by body mass, total filament length, and cardiac oxygen consumption during OXPHOS (r² = 0.79). Absolute aerobic scope (AAS) is the difference between MMR and SMR and represents the capacity of a fish to perform energetically costly activities above maintenance. Variation in AAS was predicted by body mass, total filament length, and cardiac oxygen consumption during OXPHOS (r² = 0.71). Cardiac metabolism in the LEAK state was not correlated with SMR, MMR, or AAS. These results suggest that the capacity for branchial gas exchange and aerobic cardiac metabolism are linked to intraspecific variation in aerobic metabolism of fish.