In the Yodogawa River (Osaka Prefecture, Central Japan), nutrias (Myocastor coypus) have been preying upon unionid bivalves since around 2010. We tried to infer characteristics of this behavior, such as vulnerable shell size fractions and mortality rate, by sampling live and articulated dead shells monthly from September 2012 to August 2013 at the Yakumo wando (groyne field) in Moriguchi City. Between seventy and nearly ninety percent of dead Nodularia douglasiae nipponensis shells had scratches that were judged to have been made by nutrias' incisors during prey handling. The mean shell length of scratched dead shells was larger than that of live ones in every month. These tendencies were also found in the annual data for Lanceolaria grayana. By contrast, less than three percent of live unionid shells had scratches. Although it was difficult to assess the exact rates, this predation might account for the majority of mortality in N. d. nipponensis and L. grayana around the study site. Nutrias apparently prefer comparatively large-sized unionids. Scratches were found on the right shells more than the left in both N. d. nipponensis and L. grayana. This might indicate that most nutrias around the study site attack these unionids from the right side. Prompt measures are needed to prevent this mussel predation as it will have a harmful influence on the river's ecological system.
淀川のヌートリアによるイシガイ科貝類の捕食事例, および死殻から推定されるその特徴
