For marine animals with biphasic life stages, different environmental conditions are experienced during ontogeny so that physiological constraints on early stages could explain adult distributions and life history traits. The invasive and cool-temperate adapted Mytilus galloprovincialis intertidal mussel approaches the eastern limit of its biogeographic distribution on the south coast of South Africa, where it shares a habitat with the warm-temperate adapted and indigenous Perna perna mussel. As adults, the two species exhibit different metabolic regulation capacities in response to temperature. We compared the acute metabolic response to temperature between species during the post-settlement recruit stage. Aerobic respiration rates of recently settled recruits were measured monthly for 5 months for temperatures 5 °C above or below the ambient field seawater temperature at the time of collection. Unlike adults, the capacity for aerobic metabolic regulation in response to temperature differed little between species under the conditions tested, indicating a similar degree of phenotypic or developmental plasticity in response to the thermal environment. In addition, monthly variations in metabolic patterns indicate unexpectedly high plasticity in response to recent seasonal thermal history for both species.