It is commonly assumed that metabolic rate scales with body mass to the 3/4 power. However, during ontogeny, the metabolic rate of pelagic (open-water) animals often scales isometrically (in a 1:1 proportion) with body mass. This is a robust pattern, occurring in five different phyla. It can also be seen by comparing pelagic and benthic (bottom-dwelling) species within phyla, as well as pelagic larvae and benthic adults within species. High energy costs of continual swimming to stay afloat or of rapid rates of growth and reproduction in response to high levels of mortality (predation) in open water, or both, may contribute to the isometric or nearly isometric metabolic scaling of pelagic animals. The observation of isometric scaling and other kinds of metabolic scaling both within and among species suggests that metabolic scaling is not simply the result of physical constraints, but is an evolutionarily malleable trait that responds to ecological circumstances.