In the Shigenobu River Estuary (33 ° 48′ 18″ N, 132 ° 41′ 10″ E), Matsuyama (Japan), Helice tridens inhabits the mid-intertidal zone to the limit of the high tide, while H. japonica is found around the mid-intertidal zone. In addition, H. japonica is distributed further downstream than H. tridens. These differences suggest that H. japonica has a lower salinity and desiccation tolerance than H. tridens. Laboratory experiments on salinity tolerance confirmed the higher tolerance of H. tridens compared to H. japonica. Helice tridens survived at least a week under fresh water conditions, whereas most of the H. japonica died. Helice tridens was shown to be more able to tolerate desiccation than H. japonica, through behavioral, not physiological, adaptation to dry conditions. The present results demonstrate a stronger adaptation of H. tridens to semi-terrestrial and/or freshwater environments than H. japonica.