1. Elemental composition (carbon : nitrogen : phosphorus, C : N : P) was analysed in eggs and juveniles of two crustaceans, Daphnia magna (Cladocera) and the crayfish Astacus astacus (Decapoda). Stoichiometry was also analysed for the carapace, muscle tissue, hepatopanchreas and gills of Astacus.
2. For both species the C : P ratio was significantly higher in eggs than juveniles, but there was a constant, homeostatic elemental ratio in eggs during embryogenesis (Astacus) and with different C : N : P in maternal food (Daphnia).
3. Differences in the stoichiometry of major tissue categories in Astacus suggest that there are distinct allocation strategies of elements to various somatic tissues as well as to reproduction versus somatic tissues overall.
4. There are strong ontogenetic shifts in the allocation of energy and elements in both species, as for crustaceans in general. During maturity there may be a trade-off with regard to the allocation of C, N or P to somatic or reproductive tissue, and poor food quality (high C : P in food) could pose other constraints on reproductive capacity than does food shortage (low C).
5. Egg production may be at least as sensitive to low P as is somatic growth and could result in a marked decrease in overall population growth rate more severe than would be expected from individual growth rate alone.