ヤマトカワニナの垂直分布週年變化と生活史概觀

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1. Melanoides(Semisulcospira)niponicus(Smith)(Fig.1), a fresh-water snail found at the stony beach of Lake Biwa near the Otu Hydrobiological Station, manifests regular seasonal vertical migration. Generally speaking, the snail stays in the shallow water from mid spring to early autumn and passes winter in the deeper water. 2. The snails were collected on 21/I'36, 7/III'35, 15/VI'35, 17/VIII'36, 31/X'35 and 25/XI'35, at six stations near the laboratory, with the depth of 20cm, 30cm, 40cm, 70cm, 100cm and 130cm respectively(Figs.2, 3)and thrice at each depth, by means of a scooping net(Fig.4), in which the bottom pebbles(Fig.3)were also lifted. 3. Both the vertical distribution and the migrating behaviour of the smaller individuals(less than 6mm in diameter)differ from those of the larger individuals. The younger snails are most flocked at a place 20cm deep from the mid spring to the early autumn, while the larger ones are found most frequently at a place 30cm deep from mid spring to mid summer. The migration of the lager snails to the deeper water begins in the early summer and in the late summer(in the middle of August). The maximum density of population is already found at the depth of 70cm, where they over-winter, but the duration of migration covers about 4 months ending as late as in Novenber. The migration of the younger shells to the deeper water seems to take place a little later, probably in the early autumn, and in winter they show a tendency to occupy a more or less shallower places than the larger snails. In the larger snails, the shoreward migration begins in the early April when the water temperature rises up to about 10℃ and it is completed within about one month. In this case, too, the migration of the smaller snail seems to commence somewhat earlier than that of the larger individual(Figs.5, 6, 7). 4. Of the environmental factors, temperature may be a controlling one, 10℃ being apparently the critical temperature as to the annual migration, at least in the case of the shoreward migration(Fig.8), while some other environmental factors suth as dissolved organic substances, dissolved oxygen and the disturbance of the water caused by the waves, etc. seem to have less intimate relations with the migration(Tables 1, 2). Furthermore, the breeding habit of the snail, too, may concern with the annual migration. 5. A brief account is given as to the fact of the diurnal rhythmic activity of the snail. 6. General outline of the life history of this snail is given diagramatically in Fig.9(refer to Fig.8 and Table 3).
1937-02-15

ヤマトカワニナの垂直分布週年變化と生活史概觀

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