Cristaria plicata

Japanese: カラスガイ(烏貝) - カラスガイ（英語表記）Cristaria plicata

A large, black bivalve mollusc (illustration) that lives in lakes and marshes. Its distribution extends from China to Siberia, the Korean Peninsula, and Japan. The shell is 20 cm long, 12 cm high, and 6 cm bulge, but large specimens can exceed 25 cm in length. The surface is yellowish-green when young, but turns black as it grows. It was given the name "karasu gai" (black shell) from its color. In the Lake Biwa region, the thick-shelled ikechogai (illustration), which is the mother mollusc for freshwater pearl cultivation, is called "ongai," while this thin-shelled species is called "mengai," and they are considered to be male and female.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

湖沼にすむ大型の黒いイシガイ科の二枚貝（イラスト）。分布は中国を中心にシベリア，朝鮮半島，日本に及ぶ。殻の長さ20cm，高さ12cm，膨らみ6cmになるが，大型の個体は長さ25cmを超える。表面は幼貝のときは黄緑色であるが成長とともに黒色となる。その色からカラスガイの名がついた。琵琶湖地方では淡水真珠養殖の母貝となる殻の厚いイケチョウガイ（イラスト）をオンガイというのに対して，殻の薄いこの種をメンガイと呼んで，雌雄と考えていた。

出典　株式会社平凡社世界大百科事典第２版について　情報

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