|Budget Amount *help
¥2,400,000 (Direct Cost : ¥2,400,000)
Fiscal Year 1999 : ¥700,000 (Direct Cost : ¥700,000)
Fiscal Year 1998 : ¥800,000 (Direct Cost : ¥800,000)
Fiscal Year 1997 : ¥900,000 (Direct Cost : ¥900,000)
Since 1979, on the Tomioka Bay sandflat in the Ariake-Sound estuarine system, Kyushu, Japan, following the population explosion of the callianassid ghost shrimp, Nihonotrypaea harmandi (Decapoda), its constant high density has been maintained until 1994. From 1995 the population began to decline, with the density in 1999 returning to that in 1979. This would be due to the predation by a stingray, Dasyatis akajei, which has suddenly increased since 1995. The ghost shrimp was a powerful bioturbating key species of the benthic community and has negatively affected many species, including another community key species, the gastropod Umbonium moniliferum. Although the gastropod population went extinct in 1986, it has recovered from 1997. Local populations of any species metapopulation of intertidal sandflats in the present estuarine system, including the Tomioka Bay sandnflat population, can interact through exchanging their larvae. Concerning the source-sink relationship of the local populations between the Tomioka Bay sandflat and other sandflats in relation to the oceanographic conditions of the estuarine system, it was revealed that (1) in the case of U. moniliferum with a 2-d larval period, the relationship is reciprocal due to a swift larval transport due to the strong tidal currents, and the recovery of the Tomioka Bay sandflat population owes its larval source to the extant populations in Ariake Sound, and (2) in the case of N. harmandi with a 3-4-wk larval period, the Tomioka Bay sandlat population can only be a source due to the time-averaged water current field. For the recent population explosion of the ghost shrimp in the entire estuarine system, an increased survival of larvae was suggested as a principal causal factor (e.g. a broad-scale bottom-up effect due to eutrophication). Concerning the recent widespread increase in the stingray population, its top-down effect on each local benthic community was also suggested.