Field observation of natural and artificial spawning sites of horseshoe crab Tachypleus tridentatus was conducted at Sone tidal flat in Kita-Kyushu City. Natural spawning site is located at the sandy beach at the comet tail extending landward from the lee side of Mashima island on tidal flat. Beach profile and grain size distribution of the spawning site were measured. Artificial nourishment was carried out in front of the gently sloping coastal revetment. Spawning of horseshoe crab was confirmed on this nourished beach, but nourished sand on the artificial beach was carried away by longshore sand transport under wave action. In order to create a stable spawning site for horseshoe crab, it is necessary to build groins to stabilize sandy beach.
