Faunal response of bivalves and gastropods after the construction of the dyke for reclamation was compared among the case studies in Japan (Isahaya Bay) and Korea (Saemangeum and Sihwa). After the isolation of Isahaya Bay, aspects of water quality suddenly changed, and fauna of bivalves and gastropods was drastically replaced. In March 1997, before the dyke was completed, 15 species of marine bivalves and gastropods were collected in large numbers. These species were still alive in May 1997, but most of them had died off by August 1997. However, an introduced species, Potamocorbula sp. cf. P. laevis that was not found prior to the isolation of this bay, replaced the pre-isolation bivalve community. This species also increased in Saemangeum and Sihwa, western Korea after the dyke construction. Therefore, this species can survive and multiply alone through large environmental changes such as isolation. Fossils of Potamocorbula sp. were also found from several horizons in Pleistocene and Holocene deposits in Japan and China. These fossils have common features; 1) most specimens are less than 10 mm in shell length, 2) only this species densely occur in a shell bed, and 3) some marine bivalves occur in lower horizons of the shell bed. We observed that Potamocorbula sp. could survive and multiply alone after isolation, and we therefore suggest that aggregations of Potamocorbula shells in Pleistocene and Holocene deposits represent similar isolation events in the past.
