Species delimitation of Planosiphon gracilis morphospecies (Scytosiphonaceae, Phaeophyceae) from Japan and the description of Pl. nakamurae sp. nov.

Molecular phylogenetic analyses based on mitochondrial cox1 and cox3 and chloroplast rbcL gene sequences revealed that the morphospecies of the brown alga Planosiphon gracilis (Scytosiphonaceae, Ectocarpales) from Japan is a complex of three distinct lineages (lineages I-III). Specimens from these lineages had the typical morphology of Pl. gracilis (i.e. flattened, linear, hollow, and gregarious erect thalli); thus, morphological discrimination among them was difficult in many cases. We observed large genetic divergences among the three lineages (c. 10% in cox1 and cox3). Moreover, we observed a reproductive isolating barrier between lineages I and II, despite the wide overlap in their geographic distribution. Our data, therefore, suggest that these three lineages are distinct species. Since lineage I included the holotype specimen of Pl. gracilis, this lineage was attributed to Pl. gracilis. Lineage III was phylogenetically close to Pl. complanatus from Canada with a 4.1% divergence in cox1. For lineage II, a new species name, Pl. nakamurae sp. nov., was proposed. This new species was also found in Argentina.