Widely distributed red algae often represent hidden introductions, complexes of cryptic species or species with strong phylogeographic structure.
Despite studies suggesting that most seaweeds are poor dispersers, many red algal species are reported to have circumglobal distributions. Such 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 reportedly circumglobal species must be questioned. In this study, we analyzed molecular data sets (rbcL gene) of nine species in the Rhodomelaceae for which samples were available from widely spaced geographical locations. Three overall patterns were identified: (i) species showing strong phylogeographic structure (i.e., phylogenetic similarity correlates with geographical provenance), often to the point that populations from different locations could be considered as different species (Lophosiphonia obscura, Ophidocladus simpliciusculus, Polysiphonia villum, and Xiphosiphonia pinnulata); (ii) species with a broad distribution that is explained, in part, by putative human-mediated transport (Symphyocladia dendroidea and Polysiphonia devoniensis); and (iii) non-monophyletic complexes of cryptic species, most with a more restricted distribution than previously thought (Herposiphonia tenella, Symphyocladia dendroidea, and the Xiphosiphonia pennata complex that includes the species Xiphosiphonia pinnulata and Symphyocladia spinifera). This study shows that widely distributed species are the exception in marine red algae, unless they have been spread by humans.