Studies on life cycle of a protogynous crustacean, Gnorimosphaeroma naktongense

研究了青森县koikuchino湖淡水区和新潟县Agano河淡水区两种雌雄同体甲壳动物gnimosphaeroma naktongense的分布和生活史。本种主要分布在咸淡水沿岸的湖泊和河流中，但它们正在进入日本海一侧的纯净淡水部分，例如它们生活在新泻县的Jimoto泉中。但在太平洋一侧，像北上河（岩手县）这样的上游地区却没有发现。认为生活在日本海一侧咸淡水域的中华黄芪是太平洋一侧中华黄芪的一部分。nakotngense在咸淡水中有两代，夏季一代从6月开始，秋季一代从10月开始，在淡水中有一个清晰的一代从6月开始。详细的性别变化模式分析表明，它们在第一个繁殖季节（约1年）为雌性，在它们的后代，蝠鲼幼虫释放后，由雌性变为雄性，大约1年后有下一个繁殖季节。它们表明了明显的两性二态性，雄性的体重比雌性大，而且在许多情况下，它们倾向于体型较大的雄性与体型较大的雌性交配，或者是一种体型匹配的交配系统。实验表明，这种现象一直存在的原因是，在交配前的保护中，一个较大的雄性选择了一个较大的雌性，而雄性将其保护的雌性换成了一个更大的雌性，有时一只较大的雄性从一对保护的雌性中取出一只雌性。生活在淡水中的种群比生活在咸淡水中的种群产蛋数少，但两种群之间的卵数没有变化。这表明生活在淡水中的种群还没有完全适应淡水环境。

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.