Morphological variations of the seagrass species, Halophila nipponica (Hydrocharitaceae, Alismatales)

During recent field surveys in the Okinawa Islands, southern Japan, we found a peculiar seagrass growing at sandy ground 10 m in depth at one of the Islands, Ie Island (27 June 2010). The overall gross morphology of this plant was very similar to that of Halophila species, while the leaves were much narrower, up to 1 mm in width (up to 3 cm in length). The cross veins were not recognized with naked eyes and under dissecting microscope, but under optical microscope 2-4 cross veins and intramarginal vein were observed in the leaves. In cross section, the leaves were undulated each side of the midrib. To clarify the exact taxonomic status of this plant, the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA was analyzed. As the result, this plant was included in the H. nipponica clade and possessed the identical ITS sequence with the plants from Nakagusuku Bay, Okinawa Main Island having broad linear leaves, and from Ooura Bay, Ishigaki Island having elliptical leaves, typical forms in the H. nipponica clade. Our present study therefore suggests that H. nipponica has a marvelously wide range of morphological plasticity and that the gross morphology of leaves might be of little significance in specific descrimination.Proceedings of the Horiba International Conference "New Direction of Ocean Research in the Western Pacific" : Past, Present and Future of UNESCO/IOC/WESTPAC Activity for 50 years and the JSPS Project "Coastal Marine Science"Section I: Research Articles/Biodiversit