On rocky shores, wave-swept macroalgae experience dramatic and repeated wave-induced hydrodynamic forces. However, previous studies of macroalgal mechanics have shown that individual waves are not forceful enough to account for observed rates of breakage. Instead, fatigue may contribute to algal breakage, with damage accumulating over time in conditions of repeated loading. Here I examine the entire process of fatigue, from crack initiation to eventual specimen fracture, in the common red alga Mazzaella. Propensity for fatigue failure in laboratory tests varied with life history phase and species: at a given repeated loading stress, male gametophytes endured more loading cycles before breakage than tetrasporophytes, which in turn lasted longer than female gametophytes; likewise, M. splendens withstood more loading cycles at a given repeated loading stress than M. flaccida. Fatigue failure begins with formation of cracks, the timing and location of which were assessed. Cracks formed, on average, after approximately 80–90% of cycles required for failure had passed, although crack timing varied with life history phase. Also, crack formation frequently occurred in association with endophytes and female gametophyte reproductive structures, suggesting a cost of endophyte infection and a tradeoff between reproduction and mechanical survival. Comparison between laboratory and field loading conditions provides robust confirmation that fatigue breaks fronds in natural M. flaccida populations. Large, female gametophyte fronds are predicted to be most susceptible to fatigue failure in the field, whereas small, male gametophyte fronds are least likely to break.
- crack initiation
- crack growth
- life history phases
E. Carrington generously provided use of her equipment and laboratory for testing of Mazzaella splendens. M. Boller and L. Miller assisted with equipment preparation for drag coefficient measurements. M. Denny and D. Nelson provided critical advice and guidance in the design and presentation of this research. This manuscript benefited from the suggestions of J. Connor, L. Hunt, B. Grone, D. Mach, J. Mach, K. Miklasz and S. Tepler. In completion of this study, I received support from the Myers Oceanographic and Marine Biology Trust Grant, the Friday Harbor Laboratories Wainwright Fellowship, a Stanford Graduate Fellowship, and a Stanford Interdisciplinary Graduate Fellowship.