Abstract:We examined the species compositions and community structures of halophytic plants, gastropods, and brachyurans in salt marshes (52 sites) in the Ariake and Yatsushiro seas of the western Japan, and assessed the abiotic and biotic environmental factors driving the community structures. Moreover, differences in community structures were compared among the three main study areas: the Ariake sea area, Yatsushiro sea area, and Uto-Amakusa area (shoreline sites located between the Ariake and Yatsushiro seas). A total of 11 halophytic plant species, 19 gastropod species (groups), and 19 brachyuran species were recorded. The average number of halophytic plant species was the highest in the Uto-Amakusa area, but the numbers of gastropod species (groups) and brachyuran species were similar among the three areas. For each halophytic plant species, gastropod species (groups) and brachyuran species, the community composition in the Uto-Amakusa area differed greatly from that in the Ariake and Yatsushiro sea areas; however, those of the Ariake and Yatsushiro sea areas were similar. The dissimilarities in halophytic plant species among the areas were driven mainly by the presence of tall, dense Phragmites australis. Moreover, the most important environmental factor that affected community structure was interstitial water salinity in halophytic plants and brachyurans, and distance from the open shoreline in gastropods.