Twice in the past, the yellowtail Seriola quinqueradiata TEMMINCK et SCHLEGEL was cultured on a large scale in the coastal waters of the Amami and Ryukyu Islands which are remote from its natural habitat, that is, the coastal waters of the Japanese main lands of Honshu, Shikoku and Kyushu. The seedlings used were juveniles reared in farms in Shikoku and Kyushu and the foods used were the same frozen trash fishes as were commonly used in farms in the main lands.
Unexpectedly, in both cases most of the cultured yellowtails were seriously affected by kudoasis. All affected fish lost entirely their commercial value, because numerous white spherical or elliptical trophozoites, mostly 1-5 mm in size, of Kudoa sp, were formed in the skeletal muscle of all parts of the body.
Field studies made by the present authors for the past 3 years revealed the fact that coral fishes in Amami and Ryukyu waters, that is, Abudefduf sexfasciatus, A. vagiensis, Chromis isharai, C. notatus and Chrysiptera assimilis (Percida : Pomacentridae), occasionally carried trophozoites of the same Kudoa sp. in their skeletal muscle, though in these fishes the number of trophozoites in an individual fish was small, one in almost all cases. A scanning-electron-microscopic examination demonstrated that the spores of Kudoa collected from yellowtails were identical in morphology with those from coral fishes. A new name Kudoa amamiensis was tentatively given to this Kudoa.
Through various field studies and experiments a hypothesis is possible that the source of infection of kudoasis in yellowtails cultured in Ryukyu and Amami waters may be the above-mentioned coral fishes and yellowtails may become seriously infected by eating coral fishes bearing sporulated trophozoites of the myxosporidia which happen to swim into net pens for yellowtails.
