The habitat, morphology, and propagation characteristics were examined on ecklonian kelps in Nishinoshima coast of Oki Islands that were difficult to identify. In addition, cultivation experiments and DNA sequencing on ITS-1 region were conducted. The data obtained were compared with those of Ecklonia stolonifera and E. kurome, and taxonomical discussion was done. The stolon of E. stolonifera was characterized by its small number of branches and cteno-rootlet on the back of the root. The root of E. kurome was characterized by multiple branching and rootlet at the tip of the root. The unidentifiable kelps were divided into two types (AK-1 and AK-2) according to the presence or absence of shoot at the tip of the stolon. Furthermore, plants without shoot (AK-2) were divided into two subtypes (AK-2(1) and AK-2(2)) based on the difference of morphology of holdfast. By morphological principal components analysis and sequence comparison, plants that have shoot (AK-1) and plants that have a root with the feature of E. stolonifera but have no shoot (AK-2(1)) were considered to be E. stolonifera. Furthermore, it was inferred that plants that have roots with the feature of both E. stolonifera and E. kurome and have no shoot (AK-2(2)) were sporophytes from natural crossbreeding between two ecklonian species.
