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. The intermediate position of cladocerans cific functions of clearance, assimilation, and in the food chain and their amenability to respiration rates from various sources into experimental manipulation have stimulatgeneral models of size-specific net energy ed a long tradition of research on their enintake (Threlkeld 1976; Hall et al. 1976; ergetics. A general assumption of this work Lampert 1977a; Lynch 1977). Such models is that the difference between short term have been used to generate expected patmeasures of assimilation and respiration can terns of size-specific variation in competibe extrapolated to provide more long term tive ability, vulnerability to starvation, and estimates of secondary production (growth life history expression and evolution. The plus reproduction). Unfortunately, research accuracy of the predictions of these models in this area has progressed in a rather disis obviously a function of the validity of the organized fashion. With the exception of the assumptions that interspecific variation in early work by Richman (1958) and Schindthe size specificity of energetics is not great ler (1968), virtually no one has simultaand that the pooling of data from different neously measured assimilation and respistudies (often with very different experiration. Controlled interspecific comparisons mental settings) is not of major conseare even rarer. Burns (1969), Downing quence. A third problem is the possibility (1981), and DeMott (1982) provide some that differences between short term assiminformation on clearance rates, but almost ilation and respiration measurements may no comparative data are available for asnot in fact be reflected in realized growth similation or respirationthe two energetic and reproduction. subcomponents that ultimately determine In order to examine these problems we the net energy intake of individuals. raised four species of Daphnia in compaThis lack of data has not discouraged sevrable environments and measured their sizeeral individuals from combining size-spespecific rates of feeding, assimilation, respiration, growth, and reproduction. Our results reveal that the complexities adSupported in part by NSF grant DEB 79-1 1773 to dressed in the previous paragraph are inM. Lynch and a fellowship from the Max-Planck-Indeed real and that future work in this area stitut to L. J. Weider. should proceed with much more attention * Permanent address: Department of Ecology, Etholto detail than it has in the past. We thank ogy, and Evolution, University of Illinois, 606 E. HeaB. Heckt and K. Spitze for assistance, and ley St., Shelford Vivarium, Champaign 6 1820. Permanent address: Department of Biology, UniW. DeMott and J. Jacobs for helpful comversity of Windsor, Windsor, Ontario N9B 3P4. ments.