TOPOGRAPHIC FEATURES OF A SANDSPIT AND EBB TIDAL FLAT ON NATSUME COAST IN KASAOKA CITY IN VIEW OF HABITAT OF HORSESHOE CRAB TACHYPLEUS TRIDENTATUS

Topographic features of a sandspit and ebb tidal flat on the Natsume coast located at Konoshima Channel in Kasaoka City in Okayama Prefecture were investigated through field observation. The sandy beaches of this coast have been utilized as a spawning site of horseshoe crab Tachypleus tridentatus but recently large beach changes have been observed, narrowing the horseshoe habitat located at the downcoast of the spawning site. The beach changes were triggered by waves produced by ship navigation in narrow, funnel-shaped channel. A measure to stabilize the sand spit is proposed in order to prevent diffusion of sand over the ebb tidal flat.