大分県沿岸の豊後水道で養殖されていたクロマグロThunnus orientalis 1尾の腹腔に，生時の体色が薄小豆色で全長が34mmにも達する大型吸虫1個体が寄生していた。この吸虫は，その形態的特徴からヒルディネラ属の1種Hirudinella sp.に同定された。ヒルディネラ属吸虫類のクロマグロへの寄生は本報告が最初である。寄生魚の外観に異常はみられなかったが，寄生虫周辺の内臓の表面は黒色液で被われていた。クロマグロは養殖される前に，この寄生虫の感染を海洋で受けた可能性が高い。

A trematode identified macroscopically as Hirudinella sp. was found free in the body cavity of a young individual (197 mm standard length) of Pacific bluefin tuna, Thunnus orientalis (Temminck and Schlegel, 1844), cultured in coastal waters of the Bungo Strait (western North Pacific Ocean) off Oita Prefecture, Kyushu, western Japan. When found, the trematode was pale purple in color and as big as 34 mm long. This finding represents the first record of Hirudinella sp. from T. orientalis. The infected fish showed no external disease sign, but part of the surface of its visceral organs around the trematode was black-colored. The fish probably had become infected by the trematode in the ocean before it was farmed.