In crustaceans that live in seasonal environments and produce several broods during an annual cycle, there are generally seasonal patterns in reproductive traits. Thus, females are able to modulate the trade-off between per offspring investment and quantity of eggs produced in order to maximise the fitness of their offspring in response to the environmental conditions they will face during their development. We collected females of Palaemon serratus (Pennant, 1777) that had just laid eggs during the entire laying season from December 2021 to November 2022 on the coast of Seine-Maritime (France). We measured their fecundity, biomass per egg and the fatty acid composition of the eggs. The largest females laid their eggs from December to March (“winter laying”) and from May to July (“summer laying”), while the smallest females laid from January to August. Fecundity and in general biomass per egg increased with the size of the females. The FA concentration of eggs decreased throughout the laying season and winter eggs were more concentrated in essential PUFAs than summer eggs. In summer, the old females laid eggs with the same biomass as those laid by the young females, enabling them to significantly increase their relative fecundity. All these trends are consistent with the environmental conditions encountered by future embryos and larvae. These results provide a clear illustration of the maternal effects and trade-offs between reproductive traits observed in decapods.