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Abstract River estuaries are critical aquatic environments characterized by large environmental gradients in their water quality, riverbed material, and microtopography in the longitudinal and transverse directions. Molluscan fauna are sensitive to changes in water quality or bottom sediment conditions, and include species that inhabit unique environments or have a high capacity to thrive in different habitats. The interrelations between ecology and whole physical environment of molluscan fauna have rarely been studied, especially in medium and small sized river estuaries. This study was conducted to investigate the relationship between molluscan fauna and multiple hierarchical physical factors in river estuaries, and develop a prediction model for each of the molluscan species under study. The study focused on molluscan fauna in river estuaries in medium and small size rivers, where knowledge is particularly limited. Results obtained from quadrats were classified into three groups related to physical factors, including wave and river energy and habitat scale factors. Prediction models for molluscan species indicated that physical factors affected different species at varied scales. There were species influenced by watershed scale, habitat scale, or both. Because the physical factors influenced species at different scales, prediction of potential molluscan fauna on a site is also critical, in addition to the conservation or restoration of habitats. [ABSTRACT FROM AUTHOR]

Highlights • The Japanese archipelago was classified into 15 groups using fish fauna classification. • Classification results were closely related to the process of formation of the Japanese archipelago, the ocean current in Japanese coastal waters, and the connection of the water system in the glacial age. • The classifications system used in this study enable to compare the biological integrity of fish fauna among watersheds for managing river environment. Abstract An evaluation of ecological integrity is required for ecosystem conservation and restoration. The ecological region, or "ecoregion", has been adopted as a unit of geological area to enable a comparison of the ecological integrity of different regions. The delineation of an ecological region is difficult in countries in East Asia, including Japan because of complex topographies (i.e., several peninsulas and islands) and fauna that are very finely delineated based on climate or geology. Therefore, it is important to appropriately determine the ecoregions when determining their biological integrity and comparing it among that of other ecoregions. I attempted to delineate an ecological region of the Japanese archipelago based on the similarities among fish fauna by integrating the information on fish fauna that was collected by the researchers and the national government and local governments. In addition, quantitative analyses to investigate the relationship between fish fauna classification and meteorological and geographical factors were conducted to discuss the factors that influence fish fauna classifications. The archipelago was classified into 15 fish fauna groups, and the results of these grouped classifications were closely related to the process by which the Japanese archipelago was formed, the ocean current in its coastal waters, and the connection of the water system to the glacial age. Our findings suggest that rivers within geographical areas that are different from those within the Japanese archipelago might have different fish fauna classifications based on our results and potential fish fauna depending on the characteristics of the watershed, such as the scale of the floodplain, river conflicts, or river formation process. By applying the results of our fish fauna classification, we are able to make a comparison of the biological integrity of fish fauna among different watersheds for managing the river environments or establishing conservation policies. [ABSTRACT FROM AUTHOR]