Specimens collected from Sayama-ike (Sayama Pond: Osakasayama, Osaka, Japan; sediment core) and Naktong River (Kyongsang-Nando, Korea; plankton) were examined in detail by using scanning electron microscope, and identified as Stephanodiscus vestibulis
. The species is characterized by a vestibule which is a small space at the outer opening of marginal fultoportula. In addition to the arch-like opening of vestibule reported in the original description, fan-shaped, and very narrow slit openings were observed. The number of cowlings of a central fultoportula is three according to the original description, however, some specimens possessed two cowlings were observed. In light microscopy, this species can be distinguished from similar species Stephanodiscus minutullus
and S. delicatus
by a somewhat large spot on the interfascicle near the valve margin, and not clearly observed marginal fultoportulae. It was recorded from eutrophic freshwaters of western North America, Japan and Korea.
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