| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1999: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1998: ¥1,300,000 (Direct Cost: ¥1,300,000)
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| Research Abstract |
The distribution and the life cycle of a protogynous hermaphroditic crustacean, Gnorimosphaeroma naktongense were investigated in two regions of fresh-water (L. Koikuchino-ike, Aomori Pref.) and brackish-water (Agano River, Niigata Pref.) bodies. This species was distributed in lakes along sea and rivers with brackish water, but they are entering in the part of pure fresh waters in the side of the Sea of Japan, for instance they live in a spring named as the Jimoto Spring, Niigata Prefecture. But in the side of the Pacific ocean, they were not found in the upper part of the river like as Kitakami River (Iwate Pref.). It was suggested that G. chinense which is dweller in brackish waters in the side of the Sea of Japan take a part of G. naktongense in the side of the Pacific ocean. G. nakotngense has two generations, summer generation starting from June and autumn starting from October, in brackish waters, though in fresh waters there is one clear generation starting from June. Detail analysis of sex-change pattern shows they are females in first reproductive season (about 1 year), and after they released their childrens, manca larvae, they change sex from females to males, and they have next reproductive season after about one year. They indicate clear sexual dimorphism, males are larger than female in body weight, and they have a inclination that larger males mate with larger females in many cases, or a size assorting mate system. Laboratory experiments indicated this phenomenon has been maintained in the cause that a larger male selects a larger female in precopula guarding, and a male changes his guarding female with a larger female, sometimes a sole larger male take out a female from a guarding couple. Populations inhabiting in fresh waters produced less clutch size than in brackish waters, but no change was observed in egg size between two populations. This suggests that populations inhabitiong freshwaters have not completely adapted in fresh water environments.
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