A metapopulation of unionid species consists of subpopulations connected by host fish migration and drift of individuals by river currents. We explored reproductive potential of Margaritifera laevis subpopulations in the Shubuto river system, Hokkaido, Japan. Reproductive potential was evaluated by the Reproductive Index (RI), cumulative shell length for all reproductive individuals. The distribution of the RI was rather spatially aggregated. Nine core (> mean RI) and nine satellite subpopulations (< mean RI) were found in main river, whereas one core and five satellite subpopulations were found in tributaries.

The Generalized Linear Model using the RI as a response variable was performed to identify factors affecting the spatial distribution of the RI. We tested the effects of upstream river length (URL) and the number of upstream confluences (NUC), surrogates for potential immigrant source size, as well as local habitat environments. URL and NUC had strong positive influences on the RI. It is assumed that a large number of immigrant sources exist in the upstream where juveniles may shed from host fish. These results indicate that immigration from upstream might include important processes for structuring a metapopulation of the species.