Study on the Helminth Fauna of the Japanese Wood and Field Mice, Apodemus spp., on the Tsushima and Iki Islands

As a part of an insular zoogeographical research project, an analysis was made of the helminth fauna associated with the Japanese wood and field mice, Apodemus speciosus and A. argenteus (Muridae: Rodentia) both from the Tsushima and Iki Islands in late March 1990. Six nematode species (Rictularia cristata, Syphacia frederici, S. emileromani, Rhabditis (Perodera) orbitalis (3rd-stage larva), Eucoleus sp. and Heligmonoides speciosus), 3 trematode species (Conspicuum sp., Metagonimus sp. and Plagiorchiidae gen. sp.), and 3 cestode species (Hymenolepididae gen. sp., Taenia taeniaeformis (larva) and Cladotaenia sp. (larva)) were obtained from 60 individuals of A. speciosus collected at 9 sites on the Tsushima Islands. Five nematode species (R. cristata., S. emileromani, R. (P.) orbitalis, Eucoleus sp. and H. speciosus), a trematode species (Conspicuum sp.), and a cestode species (Hymenolepididae gen. sp.) were obtained from 46 individuals of A. argenteus collected at 8 sites on the Tsushima Islands. Six nematode species (R. cristata, R. (P.) orbitalis, Eucoleus sp., H. speciosus, Heterakis spumosa and Heligmosomoides kurilensis) were obtained from 27 individuals of A. speciosus collected at 3 sites on Iki Island. The facts all of these nematodes from the Tsushima Islands are the same species with the species of mainland shows an allopatric speciations of the genera Syphacia and Helimonoides have not occurred throught 130,000-100,000 years. However, the common heligmosomid nematodes of Japanese Apodemus, Heligmosomoides spp., became extinct in this islands. Probably, the extinction is not due to an unfavorabie soil condition for a geohelminth because of the presence of H. speciosus (geohelminth) in the same mice, but due to other factors. One of the factors seems to be an accumulation and a repetition of the local extinction of Heligmosomoides spp. related to low host density throught the period of the isolation on the Tsushima Islands