*Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA; Department of Land, Air, and Water Resources, University of California, Davis, CA 95616-8627, USA; School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA; Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
Integr Comp Biol. 2008 Dec;48(6):834-51. doi: 10.1093/icb/icn069. Epub 2008 Jul 9.
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.
许多大型藻类在暴露于快速流动的水中的栖息地中具有扁平的、带状的叶片，但在受保护的地点则具有宽阔的、波纹状的“波状”叶片。我们使用巨型牛毛藻（Nereocystis luetkeana）来研究这些生态形态差异是如何产生的。在低流量的地点，牛毛藻的波状叶片仍然展开并在流动的水中不规则地飘动，从而不仅增强了对光的捕获，而且还增加了阻力。相比之下，来自快速流动栖息地的带状叶片的牛毛藻在流动的水中会坍塌成流线型束，并以低幅度飘动，从而降低了阻力和对光的捕获。野外移植实验表明，牛毛藻叶片的形状是一种可塑的特征。在实验室中，对来自不同地点的生长叶片施加模仿叶片在不同流动状态下所经历的水动力阻力的拉力实验表明，形状的变化是由机械应力引起的。在野外和实验室的生长实验中，我们绘制了波状和带状叶片的生长空间分布图，以确定这些不同形态是如何产生的。在这两种形态的牛毛藻叶片的近端部分，生长速度最高，但狭窄的带状叶片的横向生长速度低于宽阔的波状叶片。如果叶片边缘的纵向生长速度超过叶片中线的纵向生长速度，则叶片边缘的波纹会通过弹性弯曲产生。相比之下，当叶片宽度上的纵向生长速度相似时，会产生平坦的叶片。由于波纹是弹性弯曲的结果，因此具有弹性的波状牛毛藻叶片可以很容易地被推到不同的配置中（例如，叶片边缘的波纹波长可以改变，整个叶片可以扭曲成左右螺旋形状），这可能会增强叶片在流动水中的运动，减少自我遮荫并增加沿叶片表面的质量交换。
