Drag reduction in wave‐swept macroalgae: Alternative strategies and new predictions

Flexibility is key to survival for seaweeds exposed to the extreme hydrodynamic environment of wave-washed rocky shores. This poses a problem for coralline algae, whose calcified cell walls make them rigid. Through the course of evolution, erect coralline algae have solved this problem by incorporating joints (genicula) into their morphology, allowing their fronds to be as flexible as those of uncalcified seaweeds. To provide the flexibility required by this structural innovation, the joint material of Calliarthron cheilosporioides, a representative articulated coralline alga, relies on an extraordinary tissue that is stronger, more extensible and more fatigue resistant than the tissue of other algal fronds. Here, we report on experiments that reveal the viscoelastic properties of this material. On the one hand, its compliance is independent of the rate of deformation across a wide range of deformation rates, a characteristic of elastic solids. This deformation rate independence allows joints to maintain their flexibility when loaded by the unpredictable -and often rapidly imposed -hydrodynamic force of breaking waves. On the other hand, the genicular material has viscous characteristics that similarly augment its function. The genicular material dissipates much of the energy absorbed as a joint is deformed during cyclic wave loading, which potentially reduces the chance of failure by fatigue, and the material accrues a limited amount of deformation through time. This limited creep increases the flexibility of the joints while preventing them from gradually stretching to the point of failure. These new findings provide the basis for understanding how the microscale architecture of genicular cell walls results in the adaptive mechanical properties of coralline algal joints.

Section: Introductionmentioning

confidence: 99%

The extraordinary joint material of an articulated coralline alga. I. Mechanical characterization of a key adaptation

Denny

King

2016

“…frequent hydrodynamic disturbance (Puijalon et al, 2008). Algae either withstand drag by increasing tenacity and tissue strength (Koehl, 1984;Martone, 2007;Kawamata, 2001) or reduce drag by altering their size and shape in flowing water (Martone et al, 2012;Wolcott, 2007); reconfiguration is more readily achieved by algae composed of flexible tissue (Boller and Carrington, 2006a;Demes et al, 2011;Harder et al, 2004).…”

Section: Research Articlementioning

confidence: 99%

“…The drag coefficient (C d ), a dimensionless parameter that varies with algal shape (Carrington, 1990;Gaylord et al, 1994;Bell, 1999;Martone and Denny, 2008;Martone et al, 2012), of hosts with and without epiphytes declined with increasing Reynolds number (Re) (Fig. 3), a parameter that takes into account both algal size and test velocity in the flume.…”

Section: Biomechanical Consequences Of Epiphytism In Intertidal Macromentioning

confidence: 99%

“…Concomitantly, the flexible nature of these hosts may also play a role in the reduction of drag experienced by epiphytes. Flexibility enables algae to reconfigure in flowing water, becoming more streamlined (Boller and Carrington, 2006a;Demes et al, 2011;Harder et al, 2004;Martone et al, 2012). By growing epiphytically, S. ulvoidea is able to take advantage of the drag-ameliorating strategies of its flexible host.…”

Section: Biomechanical Benefits Of Epiphytismmentioning

confidence: 99%

See 1 more Smart Citation

Biomechanical consequences of epiphytism in intertidal macroalgae

Anderson

Martone

2013

Self Cite

Epiphytic algae grow on other algae rather than hard substrata, perhaps circumventing competition for space in marine ecosystems. Aquatic epiphytes are widely thought to negatively affect host fitness; it is also possible that epiphytes benefit from associating with hosts. This study explored the biomechanical costs and benefits of the epiphytic association between the intertidal brown algal epiphyte Soranthera ulvoidea and its red algal host Odonthalia floccosa. Drag on epiphytized and unepiphytized hosts was measured in a recirculating water flume. A typical epiphyte load increased drag on hosts by ~50%, increasing dislodgment risk of epiphytized hosts compared with hosts that did not have epiphytes. However, epiphytes were more likely to dislodge from hosts than hosts were to dislodge from the substratum, suggesting that drag added by epiphytes may not be mechanically harmful to hosts if epiphytes break first. Concomitantly, epiphytes experienced reduced flow when attached to hosts, perhaps allowing them to grow larger or live in more waveexposed areas. Biomechanical interactions between algal epiphytes and hosts are complex and not necessarily negative, which may partially explain the evolution and persistence of epiphytic relationships.

“…The seaweeds of wave-swept rocky shores survive, in large part, by being flexible, which provides them with the ability to bend and reconfigure when subjected to flow, thereby reducing the imposed hydrodynamic force (Koehl, 1984(Koehl, , 1986Denny and Gaylord, 2002;Harder et al, 2004;Boller and Carrington, 2006;Martone et al, 2012). The need for flexibility would seemingly pose a problem for coralline red algae, which calcify their cell walls and are therefore inherently rigid.…”

Section: Introductionmentioning

confidence: 99%

The extraordinary joint material of an articulated coralline alga. II. Modeling the structural basis of its mechanical properties

Denny

King

2016

By incorporating joints into their otherwise rigid fronds, erect coralline algae have evolved to be as flexible as other seaweeds, which allows them to thrive -and even dominate space -on wave-washed shores around the globe. However, to provide the required flexibility, the joint tissue of Calliarthron cheilosporioides, a representative articulated coralline alga, relies on an extraordinary tissue that is stronger, more extensible and more fatigue resistant than that of other algae. Here, we used the results from recent experiments to parameterize a conceptual model that links the microscale architecture of cell walls to the adaptive mechanical properties of joint tissue. Our analysis suggests that the theory of discontinuous fiber-wound composite materials (with cellulose fibrils as the fibers and galactan gel as the matrix) can explain key aspects of the material's mechanics. In particular, its adaptive viscoelastic behavior can be characterized by two, widely separated time constants. We speculate that the short time constant (∼14 s) results from the viscous response of the matrix to the change in cell-wall shape as a joint is stretched, a response that allows the material both to remain flexible and to dissipate energy as a frond is lashed by waves. We propose that the long time constant (∼35 h), is governed by the shearing of the matrix between cellulose fibrils. The resulting high apparent viscosity ensures that joints avoid accumulating lethal deformation in the course of a frond's lifetime. Our synthesis of experimental measurements allows us to draw a chain of mechanistic inference from molecules to cell walls to fronds and community ecology.