Freshwater pearl oyster - Margaritifera laevis

Japanese: カワシンジュガイ(川真珠貝) - カワシンジュガイ（英語表記）Margaritifera laevis

A bivalve mollusk of the family Marsupialidae that lives in clear mountain streams (illustration). It is so named because it sometimes has a pearl inside the shell. The shell is 11.5 cm long, 5 cm high, and 3.3 cm bulged. It is slightly thick, oblong, and weakly bulged, with a loose lower edge and a slight depression in the center. When young, the surface is smooth and yellowish brown, but as it grows it turns black and the apex of the shell becomes white as the skin falls off. The inside has a strong pearly luster. The glochidia larvae are raised in nursery pouches in the inner and outer gills, then attach with hooks to the gills, fins, or body surface of salmon or trout fish, absorbing nutrients and growing, before leaving the fish and dropping to the bottom of the water to become juveniles.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

山間の清流にすむカワシンジュガイ科の二枚貝（イラスト）。ときどき殻内に真珠をもつのでこの名がある。殻の長さ11.5cm，高さ5cm，膨らみ3.3cm。やや厚く，長卵形で膨らみは弱く，下側の縁は緩く中央部がややくぼむ。表面は幼貝では平滑で黄褐色であるが，成長すると黒色になり，殻頂のところは皮がとれて白くなっている。内面は強い真珠光沢がある。グロキジウム幼生は内外2枚のえらにある保育囊の中で育てられ，次にサケ･マス類の魚のえら，ひれ，体表などにかぎで付着し，栄養を吸い取って成長してから，魚を離れて水底に落ちて幼貝になる。

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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