Hard clam(Meretrix lamarckii) is one of the environmentally and economically valuable species at the Kashima-nada coast, which faces the Pacific Ocean. For survival of the clam, the key issues are assumed to be spawning, larva’s landing on the bottom in the shallow water area, shoreward movement of young shell, and seaward movement of adult shell although the importance of the shoreward shell movement is not confirmed. The importance of the shoreward shell movement and the physical environments for the other key issues were studied on the basis of the field data of waves, currents, water temperature, sediment size and bathymetry from 1987 to 2016 obtained at the Hazaki Oceanographical Research Station located at the Kashima-nada coast.
The appropriate water temperature for the spawning ranged between 1.0 to 2.3 degrees below and between 0 and 1.0 degree above the 30-year mean water temperature. Furthermore, the sudden increase in water temperature following a water temperature decrease induced by an upwelling is assumed to be a trigger for the spawning. One of the conditions for the larva’s landing on the bottom in the shallow water area is that the larvae are not to be transported out of the Kashima-nada coast. For that, a unidirectional longshore current is required not to last long. Other condition is likely to be that the developments of internal waves, which move the larvae offshore, are suppressed. The importance of the shoreward movement of young shell toward the shoreline was confirmed by comparing the number of the shell observed near the shoreline and a morphological parameter representing the shoreward sediment movement.

Key Words: kashima-nada, hard clam, survival, spawning, floating larvae, juvenile, water temperature, coastal upwelling, longshore current, internal wave, topographic change, longshore bar