Egg size, postembryonic yolk, and survival ability.

Neonates of many species are dependent upon a post-embryonic yolk (PEY), a residual of the energy reserve of the developing embryo. Offspring hatching from large eggs have relatively more PEY than offspring from small eggs. Among daphniid Cladocera, large species produce larger eggs than smaller species. We have found that the proportional amount of energy reserve in eggs of five species of Cladocera is similar, but neonates of the larger Cladocera are born with a greater relative amount of postembryonic yolk, as triacylglycerol, than small species. Apparently, more of the reserve is metabolized by embryos of small species. This is correlated with the higher unitweight metabolic rates of smaller animals. It has been argued that animals should produce relatively larger eggs when exposed to low or unpredictable food conditions to increase the survivorship of their offspring. The physiological constraint of greater relative energy requirements of small embryos may limit PEY and explain why offspring of larger eggs survive better in low or unpredictable food resource environments.