抄録
Cymbella janischii is an alien species of mat-forming water plants that has invaded various river systems in Japan. Recently, it is asserted that it may decrease feed for fish such as sweetfish, cause damage in water treatment facilities, or be detrimental to riverscapes. But the actual details of its spread in Japan or even of its morphological characteristics are not well known. In this survey, we first investigated Cymbella janischii in the Fuji River system using light and scanning electron microscopy. While no mat-like state condition could be found this time, the species appeared at all 5 collection locations except the upper Shio River. One difference from past surveys elsewhere was that few examples of Gomphoneis minutia were found. Apparently, the two species do not necessarily have to appear together. Secondly, specimens of C. janischii from the Fuji River system were compared with specimens collected from the upper Tama River. The Tama River specimens appear to divide into two clusters with respect to length and width and the specimens from the Fuji River match one of these clusters. It might be plausible to propose this as a new form of C. janischii. We discuss this possibility statistically.
