Indefatigable: an erect coralline alga is highly resistant to fatigue

Abstract

SUMMARYIntertidal organisms are subjected to intense hydrodynamic forces as waves break on the shore. These repeated insults can cause a plant or animal's structural materials to fatigue and fail, even though no single force would be sufficient to break the organism. Indeed, the survivorship and maximum size of at least one species of seaweed is set by the accumulated effects of small forces rather than the catastrophic imposition of a single lethal force. One might suppose that fatigue would be especially potent in articulated coralline algae, in which the strain of the entire structure is concentrated in localized joints, the genicula. However, previous studies of joint morphology suggest an alternative hypothesis. Each geniculum is composed of a single tier of cells, which are attached at their ends to the calcified segments of the plant (the intergenicula) but have minimal connection to each other along their lengths. This lack of neighborly attachment potentially allows the weak interfaces between cells to act as 'crack stoppers', inhibiting the growth of fatigue cracks. We tested this possibility by repeatedly loading fronds of Calliarthron cheilosporioides, a coralline alga common on wave-washed shores in California. When repeatedly loaded to 50-80% of its breaking strength, C. cheilosporioides commonly survives more than a million stress cycles, with a record of 51 million. We show how this extraordinary fatigue resistance interacts with the distribution of wave-induced water velocities to set the limits to size in this species.

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

“…Calliarthron cheilosporioides fronds were collected at Stanford University's Hopkins Marine Station (HMS) in Pacific Grove, CA, USA (36.622°N, 121.906°W), at the same lower intertidal site used in several previous studies (Martone, 2006(Martone, , 2007Martone and Denny, 2008a,b;Denny et al, 2013). Samples were held in chilled seawater (4°C) until used, usually within a day.…”

“…Tensile creep tests were performed coincidentally with the fatigue tests conducted by Denny et al (2013), and details of the apparatus can be found there. In short, a test sample (8-11 genicula) was excised from a basal section of a frond, and the cross-sectional areas of the end genicula were measured as described above.…”

“…In some respects, C. cheilosporioides' genicular tissue is extraordinary: it is more extensible and much stronger than typical algal tissue (including other corallines; B. Hale, Macroalgal materials: foiling fracture and fatigue from fluid forces, PhD thesis, Stanford University, 2001; Martone, 2006Martone, , 2007Janot and Martone, 2016) and it is virtually immune to failure by fatigue (Denny et al, 2013). However, in other respects, genicular tissue is rather ordinary.…”

“…Although each genicular cell is firmly anchored to the adjacent intergenicula, lateral attachment among cells (via the middle lamina) is apparently weak (Martone and Denny, 2008b;Denny et al, 2013). As a result, if one cell breaks, it is difficult for the elastic energy stored in the broken cell to be transferred to adjacent cells.…”

“…As a result, if one cell breaks, it is difficult for the elastic energy stored in the broken cell to be transferred to adjacent cells. This inability to transfer the energy required for fracture makes it difficult for a crack to propagate, rendering the joint highly resistant to fatigue failure (Denny et al, 2013).…”