Latitudinal variation in stipe hollowing in Eisenia arborea (Phaeophyceae, Laminariales)

Morphological variation is common in kelps (Phaeophyceae, Laminariales) and often results from differences in environmental conditions. Stipe hollowing, in particular, occurs in several kelp species worldwide but to date has been investigated for only a few species and primarily at local scales. Here we describe the patterns of stipe hollowing both within and among populations of Eisenia arborea across 800 km of its distribution along the west coast of North America. Our results indicate that there are large latitudinal differences among populations in both the occurrence of stipe hollowing (i.e. frequency of hollow stipes within a population) and hollowing extent (i.e. length of the hollow cavity relative to stipe length) in E. arborea sporophytes. Stipe length varied among our study locations, with the most southern population exhibiting the tallest stipes, the most northern population exhibiting the shortest stipes, and the three central populations exhibiting intermediate stipe lengths. The occurrence of stipe hollowing and hollowing extent also varied along a latitudinal gradient, with the more southern populations exhibiting both increased frequencies of hollowing and greater hollowing extents. In fact, although hollowing occurred in almost all (97%) of the sporophytes examined at the most southern location, it was absent in the sporophytes examined at the most northern location. Finally, although stipe hollowing was more common in larger stipes across all locations because of the overwhelming effect of location on the occurrence of stipe hollowing, stipe length alone is not a suitable predictor when considered across this species range.