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Costs and benefits of intertidal algal epiphytism Anderson, Laura


Organisms in the intertidal zone are regularly exposed to wave action, emersion, and competition. Competition for space may have been a factor leading to the evolution of epiphytes which have circumvented this problem by growing on other algae. Epiphytism is generally considered deleterious to hosts but, is this always true? This study explored costs and benefits of interactions between the epiphyte, Soranthera ulvoidea, and its host, Odonthalia floccosa, involving biomechanics, light acquisition, desiccation, and herbivory. Drag on epiphytized and unepiphytized hosts was measured in a recirculating water flume. Epiphytes increased drag on hosts by approximately 50% at each test velocity. Increased drag caused epiphytized hosts to be more likely to break from the substratum than hosts without epiphytes. Epiphytes experienced reduced drag when attached to hosts but sometimes broke before hosts. In fact, epiphytized hosts and epiphytes were equally likely to dislodge; this suggests that drag added by epiphytes may not be entirely harmful to hosts if epiphytes dislodge half the time, reducing overall drag on epiphytized hosts. The effects of epiphytism on light acquisition, desiccation, and herbivory were also investigated. Photosynthesis versus irradiance curves were constructed for hosts and epiphytes; saturation irradiances for both were approximately 50μmol m⁻²s⁻¹, and were not significantly different from irradiances under submerged algal canopies in the field. Thus, it was inferred that these epiphytes do not likely affect host light acquisition. Also, these epiphytes may not have arisen in response to light limitation as they reached photosynthetic saturation when exposed to light levels under other algae. When hosts with and without epiphytes were exposed to air, epiphytes doubled the time required for hosts to lose 50% of the water originally associated with their thalli. By delaying desiccation, epiphytes likely reduce physiological damage of emergent hosts. Lastly, invertebrate herbivores common to this study’s field site preferred grazing epiphytes over hosts. This feeding preference could benefit hosts by diverting herbivores away from host tissue and toward epiphytes. In sum, this study demonstrates that hosts and epiphytes often benefit by closely associating; these complex interactions could help explain the evolution and persistence of intertidal algal epiphytism.

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