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

1. The growth rate hypothesis predicts positive relationships among growth rate (mu), 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 mu, %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-mu and %P-mu 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 mu. 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.