Meeting Abstract

S7-2.4  Saturday, Jan. 5  How kelp produce blade shapes suited to different flow regimes: A new wrinkle KOEHL, M. A. R. *; SILK, W. K.; Univ. of California, Berkeley; Univ. of California, Davis cnidaria@socrates.berkeley.edu

Many species of macroalgae have flat, strap-like blades in habitats exposed to rapid water currents, but have ruffled �undulate� blades at protected sites. We used the giant kelp, Nereocystis luetkeana, to investigate how these ecomorphological differences are produced. The undulate blades of N. leutkeana from low-flow sites remain spread out and flutter erratically in moving water, thereby enhancing light interception, but also increasing drag. In contrast, strap-like blades of kelp from rapid-flow habitats collapse into streamlined bundles and flutter at low amplitude in flowing water, thus reducing both drag and light interception. Transplant experiments revealed that N. leutkeana blade shape is a plastic trait. Laboratory experiments in which growing blades from different sites were subjected to tensile forces or shading that mimicked conditions experienced by blades in different flow regimes showed that shape change is induced by mechanical stress. We mapped the spatial distribution of growth trajectories in both undulate and strap-like blades to determine how these blade morphologies are produced. The highest growth rates occur near the proximal ends of blades of both morphs. If growth rates at blade edges exceed the growth rate along the blade midline, ruffles along the blade edges are produced by elastic buckling once the blade has grown sufficiently wide. In contrast, strap-like blades are produced when longitudinal growth rates are similar across the width of a blade. Because ruffles are the result of elastic buckling, a compliant undulate blade can easily be pushed into different configurations. The ruffles along the blade edges can spring into smaller or longer wavelengths, and the whole blade can twist into left- and right-handed helices, which may enhance the blade movements in flowing water that reduce self-shading and increase mass exchange along blade surfaces.