Synopsis Many species of macroalgae have flat, strap-like blades in habitats exposed to rapidly flowing water, but have
wide, ruffled ‘‘undulate’’ blades at protected sites. We used the giant bull kelp, Nereocystis luetkeana, to investigate how
these ecomorphological differences are produced. The undulate blades of N. luetkeana from sites with low flow remain
spread out and flutter erratically in moving water, thereby not only enhancing interception of light, but also increasing
drag. In contrast, strap-like blades of kelp from habitats with rapid flow collapse into streamlined bundles and flutter at
low amplitude in flowing water, thus reducing both drag and interception of light. Transplant experiments in the field
revealed that shape of the blade in N. luetkeana is a plastic trait. Laboratory experiments in which growing blades from
different sites were subjected to tensile forces that mimicked the hydrodynamic drag experienced by blades in different
flow regimes showed that change in shape is induced by mechanical stress. During growth experiments in the field and
laboratory, we mapped the spatial distribution of growth in both undulate and strap-like blades to determine how these
different morphologies were produced. The highest growth rates occur near the proximal ends of N. luetkeana blades of
both morphologies, but the rates of transverse growth of narrow, strap-like blades are lower than those of wide, undulate
blades. If rates of longitudinal growth at the edges of a blade exceed the rate of longitudinal growth along the midline of
the blade, ruffles along the edges of the blade are produced by elastic buckling. In contrast, flat blades are produced when
rates of longitudinal growth are similar across the width of a blade. Because ruffles are the result of elastic buckling,
a compliant undulate N. luetkeana blade can easily be pushed into different configurations (e.g., the wavelengths of the
ruffles along the edges of the blade can change, and the whole blade can twist into left- and right-handed helicoidal
shapes), which may enhance movements of the blade in flowing water that reduce self-shading and increase mass
exchange along blade surfaces