The crab Halicarcinus planatus (Fabricius, 1775) is the only member of Hymenosomatidae that inhabits the southern tip of South America, and it is the only decapod species that reproduces twice a year in the Beagle Channel. In this study, we analysed seasonal variations in development duration, body size, biomass (dry weight), and elemental composition (CHN) of larvae from a population living at the southernmost limit of the species’ geographic range. Compared to offspring produced in December (early summer), larvae released in August (late winter) were on average larger and heavier, and they showed a higher carbon content (suggesting a larger lipid fraction). This pattern of intraspecific variation in larval size and biomass corresponds with minimum primary productivity in sub-Antarctic regions during winter, suggesting that large winter eggs may represent a reproductive adaptation to a regular pattern of variation in nutritional conditions, including food limitation in winter. As an additional or alternative explanation, also strong salinity fluctuations occurring in summer might reduce larval quality in the December cohort. In conclusion, seasonal variation in larval size and biomass may be related to variations in nutritional and/or physical factors.