Measurement of the carbon balance in Daphrtz 2

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 ii(B) based on observed investments in growth and reproduction indicates that the true relationship is nonlinear with a roughly constant value of P 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 in the food chain and their amenability to experimental manipulation have stimulated a long tradition of research on their energetics. A general assumption of this work is that the difference between short term measures of assimilation and respiration can be extrapolated to provide more long term estimates of secondary production (growth plus reproduction). Unfortunately, research in this area has progressed in a rather disorganized fashion. With the exception of the early work by Richman (1958) and Schindler (1968), virtually no one has simultaneously measured assimilation and respiration. Controlled interspecific comparisons are even rarer. Burns (1969), Downing (198 l), and DeMott (1982) provide some information on clearance rates, but almost no comparative data are available for assimilation or respiration-the two energetic subcomponents that ultimately determine the net energy intake of individuals. This lack of data has not discouraged several individuals from combining size-spe* Supported in part by NSF grant DEB 79-l 1773 to M. Lynch and a fellowship from the Max-Planck-Institut to L. J. Weider. * Permanent address: Department of Ecology, Ethology, and Evolution, University of Illinois, 606 E. Healey St., Shelford Vivarium, Champaign 61820. 3 Permanent address: Department of Biology, University of Windsor, Windsor, Ontario N9B 3P4. cific functions of clearance, assimilation, and respiration rates from various sources into general models of size-specific net energy intake (Threlkeld 1976; Hall et al. 1976; Lampert 1977a; Lynch 1977). Such models have been used to generate expected patterns of size-specific variation in competitive ability, vulnerability to starvation, and life history expression and evolution. The accuracy of the predictions of these models is obviously a function of the validity of the assumptions that interspecific variation in the size specificity of energetics is not great and that the pooling of data from different studies (often with very different experimental settings) is not of major consequence. A third problem is the possibility that differences between short term assimilation and respiration measurements may not in fact be reflected in realized growth and reproduction. In order to examine these problems we raised four species of Daphnia in comparable environments and measured their sizespecific rates of feeding, assimilation, respiration, growth, and reproduction. Our results reveal that the complexities addressed in the previous paragraph are indeed real and that future work in this area should proceed with much more attention to detail than it has in the past. We thank B. Heckt and K. Spitze for assistance, and W. DeMott and J. Jacobs for helpful comments.