Akoya pearl (Akoya shell) - Pinctada fucata martensii

Japanese: アコヤガイ(阿古屋貝) - アコヤガイ（英語表記）Pinctada fucata martensii

A bivalve mollusc of the family Bivalvia used as the mother shell for cultured pearls (illustration). Also called Shinjugai because it produces pearls. The shell is 7.5 cm high, 7 cm long, and bulges 3 cm. The left shell bulges more than the right. Its shape is not consistent, but is rectangular. The surface of the shell is usually dark brown with dark purple cloud-like spots and colored bands of red, green, and yellow radiating from the apex, and looks like overlapping micaceous flakes along the growth veins, but as it grows it loses these and becomes smooth, and becomes dirty as plants and animals attach to it. There is a gap at the front edge of the right shell through which the byssus comes out, and the thread is used to attach to rocks and other objects.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

養殖真珠の母貝に使うウグイスガイ科の二枚貝（イラスト）。真珠をとるのでシンジュガイともいう。殻の高さ7.5cm，長さ7cm，膨らみ3cmに達する。左殻は右殻よりも膨らむ。形は一定しないが四角形状。殻表はふつう黒褐色で黒紫色の雲状斑や赤色，緑色，黄色などの色帯が殻頂から放射状に出，成長脈に沿って雲母状片が重なったようになるが，成長するとそれらを失って滑らかになり，動植物が付着し汚れる。右殻の前縁には足糸が出る隙間があり，糸を出して岩などに付着する。

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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