Taxonomic study of two Sargassum species (Fucales, Phaeophyceae) from the Ryukyu Islands, southern Japan: Sargassum ryukyuense sp. nov. and Sargassum pinnatifidum Harvey

Species of the genus Sargassum (Fucales, Phaeophyceae) belonging to the Schizophycus group (in subgenus Sargassum sensu lato) from the Ryukyu Islands, southern Japan, were morphologically reinvestigated, and two entities were recognized: Sargassum ryukyuense sp. nov. and Sargassum pinnatifidum. The new species is characterized by: 1) compressed primary branches; 2) simple lanceolate primary and lateral leaves with an undulate and dentate margin in all stages; 3) lateral branches issuing alternate-distichously on the main branches in one plane; 4) long and flattened receptacles arranged alternate-distichously on terminal branches; 5) main and lateral branches on the stem issuing as foliar expansions in the early stage of development; and 6) vesicles with a simple coronal leaf. This new species resembles S. pinnatifidum in features 1), 3) and 5). However, it differs from S. pinnatifidum in features 2), 4) and 6). Furthermore, this new species can be distinguished from all previously described species based on the complete set of diagnostic features above and by the presence of long lingulate receptacles with short stalks. The new species is known to be endemic in the Ryukyu Islands and adjacent waters of southern Japan.