カワニナの配偶子形成と保育嚢内胚組成の季節的変化

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Seasonal changes in gametogenic activity and embryo composition in the brood pouch of the ovoviviparous freshwater prosobranch Semisulcospira libertina (Gould) were studied on the specimens collected bimonthly during a year from two different habitats in Gunma Prefecture, i. e., a channel for agriculture located at Hikita of Fujimi village at the foot of Mt. Akagi where water temperature shows fairly distinct seasonal changes and a fishpond in Hakoshima of Azuma village at the northern foot of Mt. Haruna where water temperature remains almost constant throughout the year. Histological observations on the testes revealed that in males from Hikita spermatogenesis was active from spring to summer when water temperature rises gradually, but in males from Hakoshima spermatogenesis occurred throughout the year. These results indicate that spermatogenesis is influenced by water temperature, that is, it begins with the rise of water temperature, it becomes inactive with the fall of water temperature, and it occurs continuously when water temperature is constant although it is low as at about 14℃. In all the males from both habitats, mature spermatozoa were contained in the vas deferens throughout the year. Histological observations on the ovaries indicated that oogenesis is also influenced by water temperature. In females from Hikita, oogenesis begins active from spring to summer when water temperature rises. But it does not show distinct seasonal changes as in the spermatogenesis, and occurs throughout the year with varying activities. On the other hand, in females from Hakoshima, oogenesis occurred throughout the year showing the largest development of the ovaries in March. The number of embryos in the brood pouch showed a tendency to increase with the size of mother shell, but showed a great individual variation. Snails from Hikita were larger in shell size than those from Hakoshima, having generally many embryos. In females from Hikita, the total number of embryos in each brood pouch was largest in February, having many late shelled-embryos but few early embryos. The total number of embryos decreased thereafter becoming smallest in August, but the number of early embryos was largest in August. From autumn to winter late shelled-embryos increased with the increase of total number of embryos. In Hakoshima, the total number of embryos was largest in March and smallest in September, but showed no distinct seasonal tendency. As to the embryo composition in females of this habitat, early embryos accounted for about 50% constantly throughout the year. In conclusion, it was considered that gametogenesis and embryonic development are greatly influenced by the change of water temperature. Increasing water temperature from spring to summer activates spermatogenesis and oogenesis as well as production of embryos and birth of newborns.
1992-12-31

カワニナの配偶子形成と保育嚢内胚組成の季節的変化

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