HABITATION CONDITIONS OF HERMIT CRABS IN THE VICINITY OF SEAWALL AT KEMIGAWA ARTIFICIAL BEACH

Abstract

Habitation condition of the hermit crabs in the vicinity of the seawall at an artificial beach was investigated at Kemigawa beach in Tokyo Bay. This beach is surrounded by two curved groins at south and north ends. At the northern side of the south groin, where is located inside the wave shadow zoneof the curved groin, dense habitat of hermit crabs were discovered near the mean sea level. The causes were studied by the numerical simulation of waves and nearshore currents as well as field observations. Three main causes were pointed out. At this area, direct sunshine is cut off by the seawall in summer. There exists a wave calm zone due to the wave sheltering effect of thegroin and wave induced currents always flow from outside to inside of the wave shadow zone, gathering much plankton.