Local adaptation to osmotic environment in killifish, Fundulus heteroclitus, is supported by divergence in swimming performance but not by differences in excess post-exercise oxygen consumption or aerobic scope.

Regulation of internal ion homeostasis is essential for fishes inhabiting environments where salinities differ from their internal concentrations. It is hypothesized that selection will reduce energetic costs of osmoregulation in a population's native osmotic habitat, producing patterns of local adaptation. Killifish, Fundulus heteroclitus, occupy estuarine habitats where salinities range from fresh to seawater. Populations inhabiting an environmental salinity gradient differ in physiological traits associated with acclimation to acute salinity stress, consistent with local adaptation. Similarly, metabolic rates differ in populations adapted to different temperatures, but have not been studied in regard to salinity. We investigated evidence for local adaptation between populations of killifish native to fresh and brackish water habitats. Aerobic scope (the difference between minimum and maximum metabolic rates), excess post-exercise oxygen consumption, and swimming performance (time and distance to reach exhaustion) were used as proxies for fitness in fresh and brackish water treatments. Swimming performance results supported local adaptation; fish native to brackish water habitats performed significantly better than freshwater-native fish at high salinity while low salinity performance was similar between populations. However, results from metabolic measures did not support this conclusion; both populations showed an increase in resting metabolic rate and a decrease of aerobic scope in fresh water. Similarly, excess post-exercise oxygen consumption was higher for both populations in fresh than in brackish water. While swimming results suggest that environmentally dependent performance differences may be a result of selection in divergent osmotic environments, the differences between populations are not coupled with divergence in metabolic performance.