Studies on the origin of the monogenean Neoheterobothrium hirame, the causative agent of parasitic anemia of Japanese flounder

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The monogenean Neoheterobothrium hirame suddenly appeared in Japanese flounder in the waters adjacent to Japan in mid-1990s, causing parasitic anemia in wild flounder. The impact of the parasite on the wild stocks of Japanese flounder is much apprehended. In this study, in order to clarify the origin of the parasite, several species of genus Neoheterobothrium were collected from southern flounder, Paralichthys lethostigma, and summer flounder, P.dentatus, distributed in the Atlantic waters of USA and bigeye flounder, Hippoglossina macrops distributed in Chile and compared with N.hirame.Worms collected from southern flounder were virtually identical to N.hirame in the morphology and sequences of 18S rRNA region, ITS1-5.8S rRNA- ITS2 region, and mitcondrial COI region. From the results, it is concluded that N.hirame recently invaded Japanese waters and settled there using Japanese founder as its host.According to a previous report that southern flounder was infected with N.affine originally reported from summer flounder, there was a possibility that N. hirame was synonymous to N.affine. In this study, two specimens of N.affine were obtained from summer flounder and compared with N.himame morphologically and molecular biologically. The present results indicate that the two species belong to different species, although further studies are needed for reaching a conclusion because of the small number of the specimens used in this study.Neoheterobothrium chilensis collected fom H.macrops was greatly different from N.hirame and N.affine in the morphology and the sequences of ITS1 and 28S rRNA resions. The results suggest that N.chilensis is not congeneric to the two species.