Abstract
Geomorphological features of the spawning sites of the horseshoe crab Tachypleus tridentatus were investigated in two river mouths in Moriye Bay in Oita Prefecture, one of the well-known habitats of the horseshoe crab in Japan. Its spawning sites were found at the inner sand bar of the river bend near the mouth, at the river mouth bar, and at a sandy beach surrounded by a tidal flat. The elevation and the median diameter of bed materials at the spawning sites were 0.62±0.18m (mean±SD) above the M.S.L. and ranged between 0.4 mm and 1.0mm, respectively. Dispersion patterns of the hatchlings were estimated by a float tracking experiment in the field. The result indicated that the hatchlings, which came out of the sand surface during high tide, could reach the larval habitat at the tidal flat by tidal currents in spite of their poor swimming ability. Mitigation methods necessary for construction of artificial spawning sites are proposed based on these field observations.
