We examined the change in hardness on the surface of sand according to the level saturation and unsaturated of the shore line region, and investigated the relation between the sand hardness and the occurrence of the gammaridean amphipod Haustorioides japonicus at Niigata, Japan Sea coast of the central Japan. On the shore, the hardness of unsaturated sand was 3 to 7 times higher than the hardness of saturated sand. This indicated that sand hardness changed alternately at the same point on the shore according to the saturated and unsaturated level by the seawater supply due to the effect of wave action. Laboratory experiments revealed that the number of individuals H.japonicus exposed on the surface of sand increased with the increase in sand hardness due to the decrease of the level of the saturated water in the experimetal sand substrate. These observations suggested that the increase of hardness on the surface of unsaturated sand obstructed burrowing activity of the H.japonicus in the sandy substrate, which then increased the risk of desiccation and predation by birds.
