Flexible Joints in Rigid Seaweeds: Applying Mechanical Theory to the Convergent Evolution of Articulated Coralline Algae

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Wave-swept shores are the most hydrodynamically stressful habitat on Earth. Nonetheless, they support a diverse assemblage of seaweeds, which use flexibility to reduce the area they expose to flow and to assume streamlined shapes. The role of flexibility in the evolution of algal design has been difficult to demonstrate, however, because few seaweeds are fossilized. In contrast, coralline algae reinforce their cell walls with calcium carbonate, and therefore have an extensive fossil record. In three separate instances, corallines evolved "joints," which gave flexibility to their otherwise rigid fronds. This thrice-evolved mechanical innovation has been highly successful, and present-day jointed corallines thrive in wave-swept environments. This project uses biomechanical theory to examine the basis for, and implications of, this apparently convergent evolutionary innovation. At the organismal level, experiments manipulating the stiffness of algal fronds will document the role flexibility plays in modulating wave forces. Engineering theory on optimal design will allow the determination of how closely these seaweeds approach optimality. At the tissue level, measurements of the mechanical properties of joints will reveal if the three coralline lineages have converged on a common set of material properties, and how these properties contribute to survival. Electron microscopy and a variety of chemical tests will explore the molecular structure of coralline cell walls and how these molecules are arranged to produce joints' unusual mechanical properties. Together, these measurements provide a unique look at how evolution affects mechanical design in the dynamic environment where sea meets shore. Laboratory techniques developed here will be used by graduate students and the PI to teach principles of ecology, evolution, and biomechanics to Stanford undergraduates and as part of an intensive graduate course, which to date has been attended by students from 13 countries. This grant spreads awareness and understanding of algae and evolutionary concepts by conducting field trips, labs, and open houses for K-12 students and the public.