Ligulate Desmarestia (Desmarestiales, Phaeophyceae) revisited: D . japonica sp. nov. and D . dudresnayi differ from D . ligulata

The phylogeny of ligulate and sulfuric‐acid containing species of D esmarestia , occurring worldwide from polar to temperate regions, was revised using a multigenic and polyphasic approach. Sequence data, gametophyte characteristics, and sporophyte morphology support reducing a total of 16 taxa to four different species. (1) D . herbacea , containing broad‐bladed and highly branched forms, has dioecious gametophytes. The three other species have monoecious gametophytes: (2) D . ligulata which is profusely branched and, except for one subspecies, narrow‐bladed, (3) J apanese ligulate D esmarestia , here described as D . japonica sp. nov., which is morphologically similar to D . ligulata but genetically distant from all other ligulate taxa. This species may have conserved the morphology of original ligulate D esmarestia . (4) D . dudresnayi, including unbranched or little branched broad‐bladed taxa. A figure of the holotype of D . dudresnayi , which was lost for decades, was relocated. The taxonomy is complemented by a comparison of internal transcribed spacer and cytochrome c oxidase subunit I ( cox 1) as potential barcode loci, with cox 1 offering good resolution, reflecting species delimitations within the genus D esmarestia .