Species diversity of pasture grass Skeletonema in the world sea and assessment of its classification

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Seven Skeletonema species were identified at one station in the industrial harbor Dokai Bay, Japan, in October 2007 and then monthly from January 2008 to December 2009 by morphological scanning electron microscopy observations and molecular analyses of mainly the large subunit (LSU) rDNA. We refer to one species identified as S.dohrnii using LSU rDNA, but as S.marinoi using the small subunit (SSU) rDNA and consequently we use the term S. marinoi-dohrnii complex. This is the first time that S.ardens, S. costatum s.s., and S.pseudocostatum have been recorded in Japan and that S.menzelii and S.tropicum have been identified by molecular methods in Japan. The S. marinoi-dohrnii complex was isolated with high frequency all year-round, and S.japonicum was also isolated with high frequency but not so in summer. S.tropicum was isolated from September to December, but S.ardens and S. costatum s.s. were isolated only when the water temperature exceeded 20℃. S.pseudocostatum bloomed just once, in summer, but S.menzelii was isolated in May and again in October. The continuous year-round occurrence of S. costatum s.1. in the bay is supported by the succession of these seven species. Six of the species, except S. menzelii, were important components of algal blooms in the bay. One to four species of Skeletonema were isolated every month. Monthly species diversity was higher when S.ardens, S.costatum s.s., and S.tropicum that are usually associated with tropical or subtropical waters, were isolated. In general, species diversity of the genus Skeletonema was very high in Dokai Bay.