New record of Sargassum denticarpum Ajisaka (Fucales, Phaeophyceae) from Iriomote Island (Ryukyu Archipelago, Japan)

The brown macroalgal genus Sargassum forms beds around the Japanese coast; these beds are important for marine ecosystem functioning and fisheries. In recent years, a northward shift in the distribution patterns of southern Sargassum spp. has been postulated as a result of warming in the western Pacific. To examine this theory, we surveyed species richness and diversity of genus Sargassum algae around the Ryukyu Islands, Japan, which is the northern limit of selected Sargassum species native to tropical and subtropical regions. We collected from Iriomote Island (Ryukyu Archipelago) specimens of a species resembling Sargassum denticarpum, which is endemic to Vietnam. The specimens had the following morphological characteristics in common with S. denticarpum: (1) terete or slightly compressed primary branches, (2) simple lanceolate primary and lateral leaves with dentate margins in all stages, (3) spherical-to-elliptical vesicles with or without spines and wings, (4) monoecious, and (5) compressed receptacles with spines on the margins. In addition, internal transcribed spacer 2 (ITS2) sequences of the Ryukyu specimens and of S. denticarpum collected in Vietnam clearly showed that the two are closely related. On the basis of these morphological and molecular characteristics, we present the first report of S. denticarpum in Japan.