The productivity of molluscs in Japanese lakes is, in general, much smaller than that of northern German and subalpine lakes in Europe. Japanese lakes also lack the shell zone which is so prominent a feature in many European lakes. The following facts seem to be important causes of these two phenomena. 1. The productivity of benthic animals as a whole is much smaller in Japanese lakes than European lakes, and the scarcity of molluscs in the former lakes is one of its inevitable sequences. The low productivity of extremely oligotrophic volcanic lakes reduces the mean value of the bottom productivity in Japanese lakes. 2. European and North American lakes harbour greater variety of molluscan species than Japanese lakes, and consequently various ecological habitats are occupied by molluscs better fitted to them. 3. The eurybathic molluscs such as Valvata, Bithynia and Pisidium are lacking or scarce in Japanese lakes south of Honsyu, where molluscs are restricted to the littoral and sublittoral zones, while the profundal zone remains almost uninhabited by molluscs. The high water temperature may be a limiting factor for the distribution of these molluscs in southern lakes. The lakes in higher latitudes of Japan as those in the Kurile Islands, Saghalin and Hokkaido have eurybathic molluscs and their molluscan productivity is nearly as high as that of European lakes. 4. The water of most Japanese lakes is weakly acid and poor in calcium content. These two facts may also have important significances on the low molluscan productivity. 5. The Ca-oligotrophy of the water and the low molluscan productivity are responsible for the lack of the shell zone in Japanese lakes.

View full abstract