Diaz-Tapia P., Maggs C.A., Macaya E.C. & Verbruggen H.
Widely distributed red algae often represent hidden introductions, complexes of cryptic species or species with strong phylogeographic structure
Journal of Phycology: submitted


Despite studies suggesting that most seaweeds are poor dispersers, many red algal species are reported to have circumglobal distributions. These wide reported distributions have mostly been based on morphological identifications, but molecular data have revealed a range of issues with morphologically defined species boundaries. Consequently, the real distribution of such circumglobal species must be questioned. In this study, we analysed molecular datasets (rbcL gene) of 12 species in the Rhodomelaceae for which samples were available from widely spaced geographical locations. Three overall patterns were identified: 1) species showing strong phylogeographic structure, often to the point that populations from different locations could be considered as different species (Ophidocladus simpliciusculus, Lophosiphonia obscura/Polysiphonia hemisphaerica/P. boldii, P. villum and Xiphosiphonia pinnulata); 2) non-monophyletic complexes of cryptic species, most with a more restricted distribution than previously thought (X. pennata/X. pinnulata/Symphyocladia spinifera and Herposiphonia tenella); 3) species with a broad distribution that is explained, in part, by human-mediated transport (P. devoniensis/P. kapraunii and S. dendroidea). This study shows that widely distributed species are the exception in red algae, unless they have been spread by humans.

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