MEPS 222:109-118 (2001)  -  doi:10.3354/meps222109

ABSTRACT: Biomechanical models of red algae have been developed that make predictions about blade survivorship based on tissue strengths, drag coefficients, and blade surface areas. The first 2 objectives of the present field study were therefore to examine actual survivorship of genets (i.e. holdfast + blades) from wide- and narrow-bladed species of Mazzaella G. DeToni f., as well as to compare blade survivorship to predictions of their survival from a previous biomechanics study of the 2 species. The third and fourth objectives were, respectively, to determine the most frequent break location in the genet, and finally to ascertain if surviving blades are likely to be reproductive. Low intertidal transplant sites were established at a wave-sheltered site where the wide-bladed form of Mazzaella splendens (Setchell et Gardner) Fredericq occurs, and in a high wave impact habitat of the narrow-bladed Mazzaella linearis (Setchell et Gardner) Fredericq. Short and long blades size classes (SC1, SC2, respectively) were included for each species because separate population sampling established that SC1 blades are almost never reproductive, whereas SC2 blades are potentially reproductive. Within the high wave impact transplant site, genet survival of experimental (i.e. transplanted to site of the other species) M. splendens was initially lower than for control (i.e. transplanted within native site) M. linearis presumably due to drag on the larger M. splendens blades. As predicted by the biomechanics model, long blades of control M. linearis survive better than those of experimental M. splendens, but not as well as short blades of experimental M. splendens. However, the narrow blade of M. linearis allows it to reach a reproductively mature length, whereas the short, broken survivors of experimental M. splendens are not long enough to be reproductive. In the sheltered transplant site, genet survival for control M. splendens and experimental M. linearis was similar, but replicate M. linearis populations located higher in the intertidal were stressed (i.e. bleached) and more likely to die. Survivorship for SC2 blades of experimental M. linearis was not greater than SC2 blades of control M. splendens as predicted by the biomechanics model; survivorship of these 2 treatments was similar. For both species, the junction between the stipe and holdfast was rarely the most frequent break location as predicted by previous studies of Mazzaella and other red algae. We conclude that M. linearis is more likely to survive and become reproductive at high wave impact sites because its narrow blade can persist in the face of large hydrodynamic forces. In contrast, in a sheltered site the wide-bladed form of M. splendens is still at a hydrodynamic disadvantage relative to M. linearis, but M. splendens is able to survive long enough to produce wide, potentially reproductive blades because it is apparently more tolerant of abiotic conditions (e.g. high irradiance, desiccation) within these sites.

KEY WORDS: Biomechanics · Common garden · Mazzaella splendens · Mazzaella linearis · Reproduction · Survivorship