Shape shifting predicts ontogenetic changes in metabolic scaling in diverse aquatic invertebrates

Metabolism fuels all biological activities, and thus understanding its variation is fundamentally important. Much of this variation is related to body size, which is commonly believed to follow a 3/4-power scaling law. However, during ontogeny, many kinds of animals and plants show marked shifts in metabolic scaling that deviate from 3/4-power scaling predicted by general models. Here, we show that in diverse aquatic invertebrates, ontogenetic shifts in the scaling of routine metabolic rate from near isometry (b(R) = scaling exponent approx. 1) to negative allometry (b(R) < 1), or the reverse, are associated with significant changes in body shape (indexed by b(L) = the scaling exponent of the relationship between body mass and body length). The observed inverse correlations between b(R) and b(L) are predicted by metabolic scaling theory that emphasizes resource/waste fluxes across external body surfaces, but contradict theory that emphasizes resource transport through internal networks. Geometric estimates of the scaling of surface area (SA) with body mass (b(A)) further show that ontogenetic shifts in b(R) and b(A) are positively correlated. These results support new metabolic scaling theory based on SA influences that may be applied to ontogenetic shifts in b(R) shown by many kinds of animals and plants.