The role of fluid dynamic factors on the feeding and growth of two sympatric blue mussels was examined in a laboratory flow chamber and in a dockside flow-through chamber. The experiments involved the blue mussels Mytilus trossulus and Mytilus californianus of two size categories (similar to1 and similar to2 cm shell length) and water from the Bamfield Inlet. Larger mussels cleared more water of seston than smaller ones, and the clearance rate (CR) of M. trossulus increased with velocity (1 - 18 cm s(-1)), whereas M. californianus exhibited a somewhat continuous unimodal functional response that peaked at similar to12 cm s(-1). Although growth decreased with velocity in all experiments over the range of 1-40 cm s(-1) the growth rates of M. californianus were consistently higher than M trossulus, and this difference extended to the highest velocity. It is evident that M californianus has a fluid dynamically mediated growth advantage over M trossulus, which would explain its dominance in wave-exposed habitats. Given this finding, we conclude that fluid dynamics are important to the evolutionary ecology of blue mussels. (C) 2004 Elsevier B.V. All rights reserved.