Ontogenetic Changes in Body Shape and the Scaling of Metabolic Rate in the American Eel (Anguilla rostrata)

The body mass (M) scaling of resting metabolic rate (RMR) may vary significantly throughout ontogeny for multiple reasons that are not perfectly understood. To compare two major geometric theories of metabolic scaling, surface area (SA) theory and resource transport network (RTN) theory, we tested whether ontogenetic shifts in metabolic scaling relate to changes in body shape in the American eel (Anguilla rostrata). To do so, we compared the log-linear scaling exponents of RMR to M (b(R)) and M to body length (b(L)) in juvenile and subadult eels (glass and yellow eel life stages, respectively). Glass eels exhibited a bL > 3 and b(R) significantly < 2/3, as predicted by SA theory. Yellow eels also had a bL > 3, but their b(R) was not significantly different from 2/3 or 3/4. We hypothesize that two developmental changes contribute to b(R) being higher for yellow eels: (1) a greater reliance on branchial respiration than body-surface-dependent cutaneous respiration and (2) a lower rate of thickening during subadult growth. An ontogenetic decrease in the ratio of cutaneous to gill respiration may have increased the relative importance of the physical constraints of a single-pump, closed circulatory system on the body-size-dependent rate of resource supply to metabolizing tissues (as predicted by RTN theory) in subadult eels. Future research is needed to quantify these developmental changes and their potential mechanistic effects on metabolic scaling, especially in the elver, a critical life stage between the glass and yellow eel stages, that was not analyzed in this study.