Plankton Rotatoria in Japanese Inland Waters

Abstract

1. The present paper concerns the studies of the plankton rotifers of Japanese inland waters. For this purpose, the writer examined more than 1500 plankton samples from more than 200 localities which are distributed in almost all parts of Japan. They contained, though plankton samples, not a few number of creeping and sessil rotifers as temporal plankters. 2. With the addition of previous records, the number of Rotatorian species of Japan has become 186, with 28 varieties and forms, which belong to 4 orders, 13 families and 45 genera. They are enlisted in table III. 3. Japan is conveniently divided into 9 districts. The number of species recorded from each district is as follows: — 1) Hokkaido, 68 species; 2) the north eastern district, 72 species; 3) the central mountainous district, 122 species; 4) the eastern Pacific coast district, 60 species; 5) the central district, 158 species; 6) the southern Pacific coast district (incl. Inland sea coast), 41 species; 7) the north eastern Japan Sea coast district, 31 species; 8) the western Japan Sea coast district, 25 species; 9) the western district, 38 species. 4. Some species noteworthy from various viewpoints are described. As the majority of plankton rotifers are of universal distribution, they are commonly found everywhere. So far as the rare and unusual species are concerned, the creeping rotifers have become the main subject of study. Among the euplanktic rotifers, only a few species are mentioned to be rare in Japan, namely, Conochiloides coenobasis, Brachionus angularis var. chelonis, and Keratella javana. 5. Those which are originated either in the boreal or in the austral region are noted. Most of the rotatorian species are cosmopolitan, and they may be found, with only a few exceptions, at any longitude, if the environmental conditions are suitable. Accordingly, when the problems toward the distribution are considered, either the latitude or altitude is of more interest than the longitude is. In Japan, the southern boundaries of boreal species could be clearly observed, while the northern boundaries of austral elements were comparatively vague. 6. So far as the widely distributed rotifers are concerned, there can be seen a tendency that the wider the distribution area, the larger the occurrence rate of pelagic species. 7. The samples obtained from brackish waters were not numerous. In the present study, Brachionus plicatilis, Keratella cruciformis var. eichwaldi, and Pedalia fennica were pointed out as the representative haline rotifers. 8. Some rotifers are well-known for their morphological variations. Above all, the following species have very ample variability in morphlogical aspects; i.e., Brachionus calyciflorus, Brachionus quadridentatus, Keratella quadrata, Keratella valga and Keratella cochlearis. The last mentioned species are of most interest with respect to the difference of both the seasons and the localities. 9. The annual succession of plankton rotifers was observed at the southern basin of Lake Biwa-ko. The average changes of the important species examined during they from 1927 to 1937 were figured in the diagram.