Taxonomic revision of <i>Dictyosiphon</i> (Ectocarpales <i>s.l</i>., Phaeophyceae) from the north-western Pacific, with descriptions of <i>D. asiaticus</i> sp. nov. and <i>D. sparsus</i> sp. nov

<i>Dictyosiphon asiaticus</i> sp. nov. and <i>D. sparsus</i> sp. nov. are newly described from Japan and the north-western Pacific. Molecular phylogenies based on the mitochondrial <i>cox</i>1 and <i>cox</i>3 and chloroplast <i>atp</i>B, <i>psa</i>A, <i>psb</i>A and <i>rbc</i>L DNA sequences revealed that <i>Dictyosiphon</i> comprises at least seven lineages worldwide. Japanese <i>Dicytosiphon</i> species formed two clades with high support and were sister to a clade including specimens from the cold-water regions of the northern Pacific, the Arctic, and the north-western Atlantic. Specimens of one of the Japanese clades (= <i>D. asiaticus</i> sp. nov.) were morphologically similar to <i>D. foeniculaceus</i> (generitype described from Scotland, UK), having densely branched thalli with fine branches, and were epiphytic on <i>Scytosiphon</i> spp. and <i>Chordaria</i> spp. Specimens of the other Japanese clade (= <i>D. sparsus</i> sp. nov.) were morphologically similar to <i>D. chordaria</i>, having somewhat thicker, sparse branches with blunt tips, and were epiphytic on <i>Chordaria</i> spp. We assign the third clade distributed on both sides of the North Pacific to <i>D. sinicola</i>, which was described from Washington, USA, for the following reasons: <i>Dictyosiphon</i> specimens in the region having relatively densely branched thalli with fine tips have been identified as <i>D. foeniculaceus</i>, and <i>D. sinicola</i> was distinguished from them by its remarkably larger thallus, while anatomically they are very similar. However, <i>D. foeniculaceus</i> is not widely distributed in the region, with only one genetic lineage (species) reported, so that those two taxa are considered to be ecotypes of the same species, and <i>D. sinicola</i> is the valid name for the taxon.