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First published online April 18, 2006
Journal of Experimental Biology 209, 1678-1689 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02139

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Size, strength and allometry of joints in the articulated coralline Calliarthron

Patrick T. Martone

Hopkins Marine Station of Stanford University, Pacific Grove, CA 93950, USA

e-mail: pmartone{at}stanford.edu

Accepted 2 February 2006

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, Calliarthron fronds 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.

Key words: algae, allometry, biomechanics, breaking stress, Calliarthron, coralline algae, decalcification, drag force, geniculum, intertidal, material properties, risk

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