Ecological and paleoecological implications of the rapid increase and decrease of an introduced bivalve Potamocorbula sp. after the construction of a reclamation dike in Isahaya Bay, western Kyushu, Japan

In April 1997, the inner part of Isahaya Bay was shut off from the Ariake Sea, western Kyushu, Japan, by the construction of a reclamation dike. After this isolation, salinity suddenly changed, as did the community of bivalves. In March 1997, before the dike was completed, 11 species of marine bivalves were collected in large numbers. These species were still alive in May 1997, but most of them had died by August 1997. However, an ‘introduced’ species, Potamocorbula sp. that was not found prior to the isolation of Isahaya Bay, replaced the pre-isolation bivalve community. In May 1997, a few small specimens of Potamocorbula sp. appeared near the estuary, and many subadults appeared at most stations of the Isahaya ‘Lake’ in August 1997. Fossils of this ‘introduced species’ are also found in several horizons of Pleistocene and Holocene deposits in Japan. These fossils have common features: (1) most specimens are less than 1 cm in shell length, (2) this species occurs only in a monospecific shell bed, and (3) some marine bivalves occur below the Potamocorbula shell bed. In Isahaya Bay, 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.