The reproductive emergence of Philometroides seriolae (ISHII, 1931) YAMAGUTI, 1935(nematode) from the yellowtail, Seriola quinqueradiata, was observed in a large exhibition tank of the Ohita Marine Palace from the middle of August to the beginning of September, 1969. From the observations the following process was suggested. A matured P. seriolae comes out from the interior of the fish body through a small hole made in the skin and hungs down from the hole for a few weeks or more by Ieavng one end of the body under the skin. The part of the worm body exposed to the outside loses elasticity and color and flattens. The end of the exposed part collapsed and thus the larvae are released. and scattered about widely owing to the swimming of the host. The number of the larvae carried by a mother worm varies with the size of the worms, being about 6 to 8 millions in worms measured over 30cm in length.
An eel-farm company at Yaizu City imported a large number of elvers of the European Eel from France in the spring of 1969. Through the end of August into September eels grown to 15 to 20cm in body length died at a high mortality rate. Dying eels showed an abnormal swimming action suggestive of suffocation. Observations of eels showing such an action revealed that their gills were infected, without exception, with large numbers of Trichophrya (Fig.15), while it was rarely observed in healthy eels. As Trichophrya had never been reported in the Japanese Eel, this parasite was first suspected of being introduced from France. Afterwards a similar parasite, however, was discovered in high incidences in Japanese eels in ponds in a different district(Fig.6). This note is the first to describe the suctorian infection of the eel.
A huge number of elvers of the European Eel were imported, as seeds for culture, from France into Japan in the spring of 1969. From the end of March to the early part of May ichthyophthiriasis broke out among them in many ponds. The parasites occurred on the body and fins, but were never seen on the gills. The disease caused no mortality in any pond and disappeared late in May. Taking the fact into consideration that icthyophthiriasis has never been known to occur among the Japanese elvers, though the present authors found out afterwards one case of icthyophthiriasis amog large Japanese eels in a pond, the causative parasite was thought to have deen introduced, carried by the imported elvers, from France into Japan and to have multiplied explosively but temporarily.