Precise tuning of barnacle leg length to coastal wave action

Both spatial and temporal variation in environmental conditions can favour intraspecific plasticity in animal form. But how precise is such environmen tal modulation? Individual Balanus glandula Darwin, a common northeastern P acific barnacle, produce longer feeding legs in still water than in moving water. We report here that, on the west coast of Vancouver Island, Canada, the magnitude and the precision of this phenotypic variation is impressive. First, the feeding legs of barnacles from protected bays were nearly twice as long (for the same body mass) as those from open ocean shores. Second, leg length varied surprisingly precisely with wave exposure: the average ma ximum velocities of breaking waves recorded in situ explained 95.6-99.5% of the variation in average leg length observed over a threefold range of wav e exposure. The decline in leg length with increasing wave action was less than predicted due to simple scaling, perhaps due to changes in leg shape o r material properties. Nonetheless, the precision of this relationship reve als a remarkably close coupling between growth environment and adult form, and suggests that between-population differences in barnacle leg length may be used for estimating differences in average wave exposure easily and acc urately in studies of coastal ecology.