1. Volume, dry mass and nutrient content were measured in eggs from four species of polar caridean shrimp (Crustacea: Decapoda): Chorismus antarcticus, Notocrangon antarcticus and Nematocarcinus lanceopes from the Weddell Sea (Antarctic), and Eualus gaimardii from Svalbard (Arctic). Mean egg dry mass was positively correlated with female dry mass in all species, although this was not statistically significant in Notocrangon. 2. In all species examined (Chorismus, Eualus, Notocrangon) there was a small but significant trade-off between egg dry mass and fecundity when the effect of female dry mass on both variables was taken into account. The reason for this trade-off is not clear, but it argues against the suggestion that there should be a minimum egg size to ensure sufficient reserves with additional resources being directed to larger eggs only when food is plentiful. There was no relation between egg dry mass and female post-spawning condition in any species once the effect of female mass had been allowed for. 3. Data from intraspecific studies of polar shrimps and gammarid amphipods indicate that overall investment by the female (reproductive output) and investment per offspring (egg size) are not linked. Although selection acting over evolutionary time scales must set the overall constraints to these variables, the results presented here suggest that the class of reproductive ecology models that relate overall investment to investment per offspring are not appropriate for marine invertebrates. Furthermore, refinement of existing models will need to incorporate the trade-off between brood size and egg size, and co-variation of egg size with female size.