Tidal-Flat Macrobenthos as Diets of the Japanese Eel Anguilla japonica in Western Japan, with a Note on the Occurrence of a Parasitic Nematode Heliconema anguillae in Eel Stomachs.

Dietary items of the Japanese eel Anguilla japonica inhabiting estuaries were examined by analyses of the gut (stomach and intestine) contents in two areas in Kyushu, western Japan. In a small estuary in Kagoshima Bay, where seven eel guts were examined, almost all of the dietary organisms consisted of Hemigrapsus crabs and Hediste polychaetes, both of which commonly occurred on tidal flats of this site during our survey on the macrobenthic fauna. In another large estuary in the innermost part of the Ariake Sea, where 14 eel guts were examined, 11 macrobenthic species (nine crustaceans, a polychaete, and a gastropod) were found in the gut contents, mostly consisting of mudflat-specific species. The dietary items are almost completely different not only between the two estuaries, but also among three neighboring sites within the large estuary in the Ariake Sea. These results show that Japanese eels feed on various macrobenthic invertebrates inhabiting estuarine tidal flats at each site. The variety of the prey species occupying different habitats indicates that their foraging areas extend to a wide range of estuarine tidal flats from the upper to lower littoral zones. The physalopterid nematode Heliconema anguillae was found parasitic in eel stomachs in both estuaries. The prevalence of the nematode was higher in the estuary in Kagoshima Bay (100%) than that in the Ariake Sea (43%), although the intensity in the former (4-94 nematodes per infected stomach) was comparable to that of the latter (2-96). The relationship between the nematode infection and the dietary items of Japanese eels is discussed.