Variation in Offspring Size in Daphnia and Consequences for Individual Fitness

We examine the magnitude and significance of intra-clonal variability in per offspring investment for clones from several lake populations of three species of Daphnia (Cladocera). Size at birth is shown to be a reliable predictor of per offspring investment in all three species. Although different clones and species exhibit different mean neonate sizes, we observe no strong contrasts in the amount of variability in offspring size. We measure the consequences of variable size at birth on future fitness for single clones of D. pulicaria and D. parvula, at three resource levels (including starvation). The ability of neonates to withstand starvation increases sharply with increased maternal investment. At both low and high resource levels fitness increases with size at birth, but the magnitude of this fitness difference is greatest at low resource levels. We document strong evidence for diminishing gains in fitness with increasing offspring size.