Field observations of the river mouth bars in Nakatsu tidal flat were carried out in view of aptitude of spawning site of horseshoe crab Tachypleus tridentatus. Seven locations were selected based on the topographic features relevant to spawning site. Out of these locations, larval horseshoe crab was found around the Kakise and Maite river mouth bars. Topographic features of each river mouth were summarized. Preservation of river mouth bar is necessary for the protection of rare species.
