Fresh water mussels (order Unionoida) have an important biological role as keystone species in river ecosystems. However, abundance of Unionoida are drastically decreasing, because of degradation of their habitat due to artificial river works, such as excessive riverbed excavation, channel straightening and loss of lateral connectivity between channel and floodplain.

Three Unionoida species were confirmed in the Matsuura river in Japan, and one lentic species (Anodouta lauta) has been dominantly decreased over the past 50 years. As they have low mobility, Unionoida are strongly affected by environmental change of river. On the other hand, large number of Anodonta lauta was found in Azamenose that is artificially restored floodplain wetland locates on middle stream of the Matsuura river. From the result of age estimation test, some individuals of Anodonta lauta in Azamenose are seemed to be transported from main channel to Azamenose by flood flow. But the hydraulic mechanism and behavior of mussels under flowing situation are not revealed.

In this study, we focused on the hydraulic characteristics of Unionoida in flowing water, and conducted hydraulic experiment to three Unionoida species. The result indicated that lentic species (Anodonta lauta) was easily affected by flowing water than lotic species (Unio douglasiae nipponensis, Lanceolaria grayana) that are dominant in main channel. This result indicates the reason why large number of Anodonta lauta live in Azamenose, and suggests that hydraulic characteristic of Unionoida is one of the reason that caused a decrement of Anodonta lauta in the Matsuura river.