Long and skinny support stalks (stipe) of seaweed experience lower stress forces when pulled rather than bent.

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Staying in place is not as easy as one might think, at least not among crashing waves! To remain attached to their substrate, marine macroalgae must manage strong hydrodynamic forces exerted by waves and tidal currents. Large seaweeds use different strategies to withstand these forces. For those species whose survival depends on staying put, their shape and material properties influence whether fluid forces will overcome the structural integrity of the algae’s stipe (stem-like structure) or holdfast (anchor-like structure).

Cochayuyo (Durvillea antarctica) is a seaweed that manages strong fluid forces by being flexible and stretchy. Its stipe has a flexible joint at its base that enables it to fold over and be pulled by flowing water, instead of bent. In general, if a structure is long and skinny, it will experience less stress under tension (pulling) than in bending, especially if the structure is solid. For example, the stiffer stipe on the grey weed (Lessonia nigrescens) bends in response to fluid forces, and as a result experiences roughly 800 times more stress (force per cross-sectional area) than the flexible stipe on cochayuyo algae. The amount of energy required to break either species of algae is roughly the same, however. The cochayuyo stipe absorbs energy by stretching, while the grey weed stipe resists deformation through its strength.

To explain why some algae have evolved to bend rather than be pulled by flowing water despite the increased stress, the influence of other life history factors beyond drag must be considered.

This strategy was contributed by Dimitri Smirnoff.

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“In nature, tensile systems are reasonably common and certainly diverse in biological affinities. Among lower plants, the stipes of some large algae are notable. In the Pacific Northwest some algae whose holdfasts are attached to rocks and whose fronds are on the surface have stipes between them well over 100 feet long, stipes that are loaded in tension by waves and tidal currents.” (Vogel 2003:437)

Comparative Biomechanics: Life's Physical World, Second EditionJanuary 1, 2003
Steven Vogel

Lessonia nigrescens and Durvillea antarctica are large intertidal algae abundant on Chilean rocky shores. Both species are exposed to waves with peak velocities between 1 and 6. m sec-1, and experience drag forces of the order of 0.2-20 N. Form drag is minimized on these flexible plants which are bent over parallel to the flow; however, about five times the force is required to bend a Lessonia stipe a given amount than a Durvillea stipe. Lessonia are stiffer (mean E = 22.0 MN. m -2) and stronger (mean breaking σ = 1.2 MN. m-2) than Durvillea (mean E = 3.5 MN. m-2 ; mean breaking σ = 0.7 MN. m-2), but Durvillea extend more before breaking (mean breaking ε = 0.17) than Lessonia (mean breaking ε = 0.09); hence the work that waves must perform to break either species is essentially the same (4 KJ . m-2 in 5.4 cm long specimens). In nature, both species tend to fail at flaws produced by grazing animals. Durvillea and Lessonia illustrate two basic strategies of large sessile organisms to withstand waves: being flexible and extensible, or being stiff and strong.” (Koehl 1979:634)

Journal article
Stiffness or Extensibility of Intertidal Algae: A Comparative Study of Modes of Withstanding Wave ActionJournal of BiomechanicsJanuary 1, 1979
Koehl, M. A. R.

“Consider the example of two species of intertidal seaweeds…Lessonia nigrescens and Durvillea antarctica…both species encounter the same range of flow forces, but they differ in their response (Koehl 1979). L. nigrescens stipes, which are wide near their bases and relatively stiff, are bent in flowing water. In contrast, D. Antarctica which have flexible joints at the bases of their stipes, flop over and are pulled by moving water. Calculations indicate that stresses in a bent L. nigrescens stipe are roughly 800 times greater than in a pulled D. antarctica stipe of the same dimensions and bearing the same load (Koehl 1982).” (Koehl 1984:64)

Journal article
How Do Benthic Organisms Withstand Moving Water?American ZoologistJanuary 1, 1984
Koehl, M. A. R.

Journal article
The Interaction of Moving Water and Sessile OrganismsScientific AmericanJanuary 1, 1982
Koehl, M. A. R

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