Intertidal 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.
This study grew out of P.M.'s longstanding interest in, and research of, the mechanics of articulated coralline algae; he contributed ideas and guidance throughout. K.M. conducted and analyzed the wave force measurements. S.T. conducted and analyzed the torsional experiments. M.D. conceived the fatigue hypothesis, designed and built the fatigue testing apparatus, conducted and analyzed the fatigue experiments, and wrote the manuscript.
No competing interests declared.
This research was supported by a National Science Foundation grant [IOS-1052161 to M.D.].
LIST OF SYMBOLS AND ABBREVIATIONS
- cross-sectional area of a basal geniculum
- initial cross-sectional area
- coefficient of drag
- tensile modulus
- shear modulus
- rotational second moment of inertia
- rotational moment of inertia
- rotational stiffness
- number of cycles
- frond Reynolds number
- planform area
- period of oscillation
- amplitude of oscillation
- fraction of frond area
- average strain
- instantaneous angle of rotation
- kinematic viscosity
- tensile stress
- © 2013. Published by The Company of Biologists Ltd