Fucus cottonii (Fucales, Phaeophyceae) is not a single genetic entity but a convergent salt-marsh morphotype with multiple independent origins

In low-energy salt-marsh environments, Fucus spp. frequently exhibit an atypical morphology that is characterized by the absence of an anchoring holdfast and a trend towards reduced size and buoyancy, enhanced vegetative proliferation, and often the loss of sexual reproduction. Such forms, often referred to as ecads, presumably derive from typical attached forms, but their affinities are normally difficult to establish with confidence due to their simplified and largely convergent morphology. Minute salt-marsh forms growing partially embedded in the sediment occur on Atlantic and Pacific coasts and have traditionally been recognized as an independent entity, Fucus cottonii. In this study we analyse with four microsatellite loci two F. cottonii populations from salt-marshes of Oregon (NE Pacific) and Ireland (Europe, near the species type locality), as well as local populations of other Fucus spp. that could be considered potential source populations, either directly or via hybridization. Our results show that the F. cottonii from Oregon derive from F. gardneri whereas the Irish population is closer to F. spiralis. We conclude that F. cottonii is not a coherent genetic entity, but an artificial grouping of evolutionarily independent populations that converged into similar morphologies in different salt-marsh habitats.