Abstract
A considerable number of Euphausia similis G. O. SARS were found to be infected with a parasite in the samples collected in the summer of 1969 in Suruga Bay. The same species was also parasitized by an ellobiopsid, Thalassomyces fagei (BOSCHMA), but it was rare in occurrence. The former parasite is slightly oval in shape with the longer diameter ranging between 1.0 and 1.5 mm, average diameter 1.35 mm. It infests cardiac region, rarely gastric region, of the host. It has been preliminarily identified by Drs. T. OSHIMA and T. SHIMAZU of the Shinshu University as the progenetic metacercaria of a trematode species very closely related to Pseudopecoelus japonicus; its adult was originally reported to infest several bathypelagic fishes and the encysted stage was also found in jack mackerel from Suruga Bay by YAMAGUTI (1938). The occurrence of the metacercaria in E. similis from the area may be new to science, although G. O. SARS (1885) has reported the immature forms of Distomum filiferum G. O. SARS in Nematoscelis megalops G. O. SARS and Thysanoessa gregaria G. O. SARS and the crustaceans of wide variety have been proved to serve as the second intermediate host of various trematodes. The euphausiids infected with the metacercaria were concentrated in the innermost part of the bay. This might suggest the presence of the euphausiid population which is confined to the geographical area for some period. The metacercaria may be used as a “biological tag” to trace the migratory range of the host when the life history of the parasite is elucidated. The finding of T. fagei in the area may be the first record of occurrence of the species in the western North Pacific, and E. similis is now recorded as the fourteenth known host euphausiid of the ellobiopsid.
