Meeting Abstract
S4.4 Monday, Jan. 4 Killing them softly: failure by fatigue in the wave-swept macroalga Mazzaella MACH, K.J.*; STAAF, A.V.; TEPLER, S.K.; BOHNHOFF, J.C.; DENNY, M.W.; Hopkins Marine Station of Stanford University mach@stanford.edu
Repeated force application typifies the lives of many plants and seaweeds. Winds repeatedly blow through the branches of trees; waves regularly crash over nearshore seaweeds, with algal thalli commonly breaking or tattering as a result. To date, however, much biomechanical investigation of plant and macroalgal tissues has focused on single applications of force, bending branches until they crack and pulling algal blades until they rupture. In the case of wave-swept seaweeds, such pull-to-break tests have found individual waves often not forceful enough to account for observed rates of algal breakage. I have investigated an alternative explanation for breakage of algal blades: failure may occur by fatigue, with damage accumulating over the course of smaller, repeated loadings. In laboratory tests on the red macroalga Mazzaella, I have quantified the process of fatigue, from initial formation of small cracks through eventual specimen fracture. Susceptibility to fatigue failure varied with species and life history stage. Most of the loading cycles to failure for any given specimen were associated with crack initiation, with cracks forming after 80-90% of the cycles required for failure had passed. Furthermore, a tradeoff between reproduction and crack formation, as well as a cost of endophyte infection, was discovered; cracks frequently formed in association with reproductive structures and endophytes. Extrapolation of fatigue behavior measured in the laboratory suggests an important role for fatigue failure in breakage observed in the field. Large, female gametophyte fronds are predicted most liable to fail by fatigue in the field, whereas at the other extreme, small, male gametophytes are unlikely to fracture from fatigue. An ongoing field study is assessing these predictions.
