Measurement of the carbon balance in Daphnia

Short term measures of assimilation and respiration rates of four species of Daphnia consistently lead to the prediction that the relation between body mass and net carbon intake, F(B), should be close to linear. Yet a more direct estimation of F(B) based on observed investments in growth and reproduction indicates that the true relationship is nonlinear with a roughly constant value of F maintained beyond the size at maturity. Several experiments demonstrate that our direct measures of F(B) are not greatly influenced by the experimental protocol. Hence, we conclude that short term measures of physiological parameters cannot be extrapolated to estimate growth and reproductive rates of Daphnia. Despite the existence of significant physiological differences between species, our results further reveal a conservative relation between age and the pattern of energy allocation to growth and reproduction in the genus and suggest that the evolution of Daphnia life histories is strongly regulated by one key character— the size at maturity