Biomechanics of the Attachment of the Intertidal Seaweed Fucus distichus

Benthic marine seaweeds are subjected to considerable hydrodynamic loads which may damage and dislodge them. Different forms of attachment systems assure that they remain in place, such as the attachment disc of the intertidal brown alga Fucus distichus. Discerning how such attachments function helps to understand the seaweed's ecological performance, may unravel interesting mechanical design features and in future may even inspire biomimetic attachment structures. Methodologically, the marine field setting is challenging and often limits comprehensive mechanical analysis. In this study, we present a setup that allows continuous in situ mechanical tests. We apply it to dislodge the attachment of young individuals of F. distichus (disc and stipe base), cross-validate our results with tensile tests in the lab, and present a thorough mechanical characterization of the attachment structure. Even though the stipes of F. distichus are weak (low breaking strength) and compliant (low elastic modulus in tension), they are thick and extensible and the work required to dislodge the attachment structure is primarily required to stretch the stipes in their elastic range. Discs are weaker than stipes (lower breaking strength), probably due to weak substrate cohesion rather than weak disc adhesion. This stresses the importance of the stipe broadening into a disc to provide a wide contact area and thereby a well-balanced attachment structure. Complementary analyses of thallus morphology and size distribution suggest that for young (small) thalli, the risk of dislodgement due to drag does not dictate thallus size. However, substrate cohesion at the sampled site may impose upper mechanical constraints to thallus size. Fucus distichus thalli not only resist drag with their attachment, but we illustrate in flume experiments that in water flow, thalli also bent down toward the substrate and streamline considerably, which will reduce the drag the attachment has to bear.