Adaptation versus Allometry: Population and Body Mass Effects on Hypoxic Metabolism in Fundulus grandis

Hypoxia has significant effects on organisms, from metabolic reduction to death, and could be an important evolutionary force affecting the variation among populations within a species. To determine intraspecific variation in hypoxic metabolism and the effect of body mass, we examine rates of oxygen consumption (Ṁo2) at seven oxygen concentrations among seven populations of Fundulus grandis that inhabit a mosaic of habitats with different frequencies and intensities of hypoxia. For Ṁo2, there is a significant interaction (P < 0.05) between body mass and oxygen concentrations: log10 body mass:log10 Ṁo2 slopes were steeper at intermediate oxygen partial pressures (Po2) than either normoxic or lowest Po2 (ANCOVA, P < 0.001). Additionally, the Po2crit (Po2 where Ṁo2 can no longer be maintained) was a negative function of body mass (P < 0.04). At the lowest Po2 (1.8 kPa), there was a significant difference in Ṁo2 among populations: one of the populations from environments more frequently stressed by hypoxia has greater Ṁo2 at the lowest oxygen concentrations. With few differences among populations, the most important effects were how body mass affected Ṁo2 at intermediate Po2 and the negative relationship between body mass and Po2crit. These findings suggest that an increase in body size is a useful strategy to minimize the effect of hypoxia.