Dispersal in Kelps: Factors Affecting Spore Swimming and Competency

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Abstract
The distance over which propagules can successfully colonize new sites depends on processes that increase the time they remain competent while being dispersed. As do feeding larvae, algal spores can contribute to their own nutrition (via photosynthesis) during dispersal. We explored the dispersal potential of the kelps Macrocystis pyrifera and Pterygophora californica in laboratory experiments by examining (1) how long their spores can swim, (2) the contribution of energy derived from photosynthesis to spore swimming duration, and (3) the ability of spores to germinate and attach after they stop swimming. Results indicate that under photosynthetically saturating irradiance no spores of either species can swim longer than 120 h; <10% of the spores were still swimming after 72 h. When placed in the dark, spores did not swim longer than 72 h; <10% remained swimming after 48 h. More importantly, spores did not die after they stopped swimming; most germinated in the water column and retained their capacity to produce viable sporophyte recruits. The ability of spores to attach after they stopped swimming differed between the two species; settlement density declined in Macrocystis and increased in Pterygophora. Thus, the viable planktonic stage of these algae is not necessarily restricted to the spore but may include later life history stages. These results provide biological evidence that the spores and germlings of these kelps can remain competent in the plankton for extended periods of time, which is consistent with our previous findings that their dispersal can occur over greater distances than previously thought possible.

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