Temperature is a major factor contributing to the biogeography of intertidal poikilotherms. The population dynamics of intertidal invertebrates like mussels are strongly dependent on larval supply and larval mortality. Unlike adults, which are resistant to fluctuating environmental conditions, larvae are highly sensitive to the physicochemical conditions of the pelagic zone. The effects of rearing temperature on larval and post-larval performance of 2 coexisting mussels species, Mytilus edulis and M. trossulus, were examined in this study. In the first experiment, larvae of both species were reared separately at constant temperatures of 10, 17, and 24 degrees C from D-larvae to the dissoconch stage. In the second experiment, pediveligers were reared under the same experimental conditions as the larvae for 2 wk. Survival, growth, and lipid composition (triacylglycerol [TAG] and sterol [ST] contents) were used to compare the performance of both species. The first experiment showed a species-specific thermal tolerance range, reflecting the biogeography of the species. M. trossulus larvae showed a preference for water at 10 and 17 degrees C, corresponding to the more northerly distribution of this species, whereas M. edulis preferred temperatures of 17 and 24 degrees C. Moreover, the TAG content in larvae of both species varied with temperature, supporting the hypothesis of a positive correlation between energy reserve content and the survival of mussel larvae. In contrast, the species-specific characteristics of thermal tolerance were not observed during post-larval development, suggesting that the selective effect of temperature on mussel species occurs during early ontogeny.