The species composition of the genus Skeletonema (Bacillariophyceae) was investigated in five Japanese coastal waters by analyzing large subunit rDNA D1-D3 sequences and the fine morphological structures of vegetative and germinated cells. The predominant species were S. grevillei from Main Okinawa Island Midwaters in the subtropical zone and S. dohrnii from the Ariake Sound, Toyama Bay, and Onagawa Bay in the temperate zone, and Funka Bay in the subarctic zone. S. japonicum was found with S. dohrnii in the latter four bodies of water. During the warm season, S. costatum, which can acclimate to low salinity, was found predominantly in the Ariake Sound and Toyama Bay, where the salinity was low, despite the fact that S. costatum is able to germinate from sediments over the temperature range of 10-30°C. The absence of S. costatum vegetative cells in the winter was due to lower specific growth rates of S. costatum than those of S. dohrnii and S. japonicum. During warm periods, S. grevillei and S. pseudocostatum occurred as the dominant species in Onagawa Bay and Funka Bay respectively, where salinities were usually >30. The species composition and seasonal succession of Skeletonema were closely related to temperature and salinity.
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