In 2009, one of the authors (R. Ishibashi) found a novel larval trematode of Family Gorgoderidae in the gonads of Unio douglasiae from the lower reaches of the Yodo River, Osaka Prefecture. This was the first record of trematodes from unionid mussels in Japan. We investigated the taxonomical position and life cycle of the trematode, as well as its potential effects on mussel reproduction.

Adult worms were obtained from the ureters of the common carp Cyprinus carpio. It was also the first record of a gorgoderid trematode from common carps in Japan. Morphological characteristics and molecular data showed that it belongs to the genus Phyllodistomum. It resembles P. elongatum Nybelin 1926 in Europe in morphology, first intermediate host, and the infection site of adult worms. A detailed investigation is needed to judge whether it is an introduced species or not.

The prevalence of Phyllodistomum sporocysts in U. douglasiae in the Yodo River was 0-10 % during June 2010 and December 2011. In a canal in Neyagawa City, the prevalence reached up to 44 % (4/9). It was not detected from mussels in Lake Biwa nor in Heian Shrine in Kyoto City. The gonads of infected mussels were degenerated and likely to be castrated. This trematode is one of the factors affecting population dynamics of U. douglasiae and should be monitored where it is detected.