Temperature is a key factor in determining the physiology and distribution of marine species and the projected rates of ocean warming are likely to outpace the ability of species to adapt. Phenotypic plasticity in traits correlated with fitness and survival offers an alternative, more rapid strategy for dealing with environmental changes. In this study we wanted to investigate phenotypic plasticity in two maternal traits, egg size and energy content, in relation to environmental temperature in the marine polychaete . Both these traits are important fitness components, but the relationship between the two is rarely explored empirically. collected from Pearl Harbor, HI were kept at 25°C in a common garden for a minimum of two generations and then newly settled juveniles were split into either 20 or 28°C until reproductive maturity. Gravid adults were then spawned and eggs from 20-25 females reared at each temperature treatment were collected to perform measurements of egg size and energy content, as well as fecundity. Females reared at 20°C produced eggs twice as large and had higher fecundity than females reared at 28°C, consistent with the temperature-size rule. To determine whether this difference in egg size reflected a difference in maternal investment strategies, we measured egg energy content using a dichromate oxidation technique. However, females reared at 28°C produced eggs with almost twice the energy content and energy density than females reared at 20°C. These results are counter-intuitive and suggest that temperature-driven changes in egg size do not reflect a decrease in per-offspring investment in this species. These results may be driven by either changes in egg composition or a selective strategy to avoid polyspermy while providing more energy in the egg to fuel the rapid metabolic rates of larvae.