Summer 2016

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories

First Advisor

Michael H. Graham


The extensive morphological plasticity of seaweeds has been appreciated since the first species were described, however, the costs and benefits of this strategy are not yet well understood. Studies have reported that morphological variants differ in reproduction, growth, and physiology, in addition to the ways in which they impact surrounding communities. This study examined the consequences of morphological plasticity in the intertidal kelp, Egregia menziesii, which is common along rocky shores from Baja California to British Columbia. Egregia is also considered to be a foundation species due to the dense canopy it forms and the numerous species that depend on it for food and shelter. This monospecific genus was previously described as several independent species due to variability in blade and midrib (or rachis) morphology. In central California, rachi can be either papillated or smooth, with papillated rachi being dominant north of Point Conception, and smooth rachi dominant south of Point Conception. The Monterey Bay area is home to populations of both morphological forms and this study evaluated the effects of rachis morphology on reproduction, desiccation resistance, grazing resistance, and growth patterns of Egregia menziesii. These questions were assessed using a suite of field surveys, field experiments, and laboratory experiments. The results indicated that reproductive output measured over the course of a year was not significantly different between the two morphological forms, however, some seasonal differences in the timing of reproductive output was observed. Smooth rachi appeared to have higher spore release, but lower settlement success than papillated rachi at the same site, indicating potential life-history trade-offs and a complicated interaction between morphology and environmental conditions. An obligate limpet was observed to occur at similar densities between the two rachis morphologies, however the grazing scars it created were significantly larger on smooth rachi, indicating that rachis papillations may offer some level of grazer resistance. Rachis morphology also significantly affected desiccation rates in the intertidal. Rachi with denser papillations exhibited slower rates of water loss over time, perhaps indicating a resistance to drying. This resistance to desiccation could also benefit other intertidal organisms sheltering under Egregia canopies. Evaluations of the effects of biomass loss on growth demonstrated that the intercalary meristem is much more diffuse than previously thought, providing Egregia with a mechanism to recover from physical damage that occurs along the rachis. This ability to repair and regrow following physical disturbance is a potential advantage in the high stress rocky environment. Overall, my results indicated that differences in reproduction, grazer resistance, and desiccation resistance occur among two distinct morphotypes of Egregia menziesii. While these two morphotypes may not be separate species, their unique characteristics result in physiological impacts, as well as impacts on the intertidal community.