Coupling of growth rate and body stoichiometry in <i>Daphnia</i>: a role for maintenance processes?

Abstract

Summary 1. The growth rate hypothesis predicts positive relationships among growth rate ( μ ), body RNA (%RNA of dry mass) and body P (%P of dry mass) contents. 2. We tested this within‐ and across‐species by growing five species/clones of Daphnia ( Daphnia magna , Daphnia pulex , Daphnia galeata and two isolates of Daphnia pulicaria ) with different combinations of food quantity and stoichiometric food quality. 3. Within each species, positive correlations among μ , %RNA and %P were seen and across species there was a strong association between%RNA and %P, consistent with the growth rate hypothesis. However, coupling of growth to %RNA and to %P differed for different species. In particular, the %RNA– μ and %P– μ relationships had similar slopes but considerably different y ‐intercepts (i.e.%P or %RNA at zero growth), with D. pulicaria and D. galeata having higher intercepts than D. magna and especially D. pulex . As a result of these displacements, the relative rankings of the species on the basis of %P and %RNA did not correspond to their rankings based on μ . 4. These findings suggest that within a narrow clade (e.g. the daphnids), interspecific differences in body P content may reflect not growth rate‐related RNA allocation but instead the amount of RNA required for support of maintenance processes.