Cho (goldfish fly) - Cho (English spelling) fish lice

Japanese: チョウ(金魚蝨) - チョウ（英語表記）fish lice

A small crustacean of the subclass Branchioura and family Butterfly Family that temporarily attaches itself to the surface of freshwater fish such as goldfish, crucian carp, and carp, sucking their blood and causing great harm. It is about 5 mm long. Its body is flat and disc-shaped and transparent. When it leaves the surface of the fish and swims, it sways back and forth like a falling leaf. Originally endemic to Japan, it has been introduced and is distributed in Europe, America, and other places. If a butterfly parasitizes the outside of a fish, the blood is sucked out and the fish becomes weak and dies, so it is best to remove it with tweezers as soon as you find it.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

キンギョ，フナ，コイなどの淡水魚の体表に一時的に着生して，血液を吸い大害を与える小型の鰓尾亜綱チョウ科の甲殻類。体長5mmくらい。体は扁平な円盤状をしており透明。魚の体表を離れて泳ぐときは，木の葉が落ちるときのように左右に揺れながら泳ぐ。元来，日本の固有種であるが，ヨーロッパやアメリカなどに移入され分布している。チョウに外部寄生されると，血液を吸われて魚が弱って死んだりするので，見つけしだいピンセットなどで取り除いてやるのがよい。

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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