The life history response of a clone of Daphnia pulex to nine concentrations of a defined food supply is described and combined with an energy analysis. Three features of this clone were essentially invariant with respect to food level: the length–mass regression, the durations of instars, and the size—specific fractions of net energy intake invested in reproduction. The main response to food limitation was a reduction in size—specific net energy intake and in egg size. Simple mathematical functions are fit to the data on these relationships and combined to produce a model that reconstructs the age—specific patterns of growth and reproduction quite accurately. The energy cost of molting is shown to be a major constraint on the evolution of body size in Daphnia.
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