Size, strength and allometry of joints in the articulated corallineCalliarthron

Abstract

SUMMARY Articulated coralline algae (Corallinales, Rhodophyta) dominate low-intertidal, wave-exposed habitats around the world, yet the mechanics of this diverse group of organisms has been almost completely unexplored. In contrast to fleshy seaweeds, articulated corallines consist of calcified segments (intergenicula) separated by uncalcified joints (genicula). This jointed construction makes calcified fronds as flexible as fleshy seaweeds,allowing them to `go with the flow' when struck by breaking waves. In addition to functioning as joints, genicula act as breakage points along articulated fronds. Here, I describe the allometric scaling of geniculum size, breaking force and tissue strength along articulated fronds in two species of Calliarthron. Genicular material is much stronger than tissue from fleshy macroalgae. Moreover, as fronds grow, genicula get bigger and their tissue strengthens, two processes that help them resist breakage. Within individual fronds, larger branches, which presumably experience greater drag force, are supported by bigger, stronger genicula. However, frond growth greatly outpaces genicular strengthening. As a result, Calliarthronfronds most likely break at their bases when critically stressed by incoming waves. Shedding fronds probably reduces the drag force that threatens to dislodge coralline crusts and may constitute a reproductive strategy.

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.…”

“…The mechanical response of genicula to cyclic loading was measured using the tensometer employed by Martone (2006) and Martone and Denny (2008a), and the details of the apparatus can be found there. In short, a length of basal frond was held between two clamps, one supported by a force transducer and the other attached to the moveable head of the tensometer.…”

“…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.…”

“…This is possible only because genicular material is much more extensible than the tissues of typical macroalgae; C. cheilosporioides' joints can stretch to more than twice their length while most seaweeds can extend by only 25% to 50% (B. Hale, Macroalgal materials: foiling fracture and fatigue from fluid forces, PhD thesis, Stanford University, 2001; Martone and Denny, 2008a). The strength of C. cheilosporioides' cell-wall material is comparable to that of other algae, but because cell walls form an exceptionally large fraction of its cross-sectional area, C. cheilosporioides' genicular tissue is approximately four times stronger than typical algal tissue (B. Hale, Macroalgal materials: foiling fracture and fatigue from fluid forces, PhD thesis, Stanford University, 2001; Martone, 2006Martone, , 2007P. T. Martone, unpublished data).…”

Indefatigable: an erect coralline alga is highly resistant to fatigue

“…This increased flexibility is achieved through a combination of morphology and material properties (Martone, 2006;Martone and Denny, 2008a). In general, genicula have a smaller cross-section than the calcified parts of the thallus, which, by reducing the second moment of area, facilitates bending (Denny, 1988).…”