EXTINCTION OF THE TROCHID GASTROPOD, UMBONIUM (SUCHIUM) MONILIFERUM (LAMARCK), AND ASSOCIATED SPECIES ON AN INTERTIDAL SANDFLAT

On an intertidal sandflat in western Kyushu, Japan in 1979, the trochid gastropod, Umbonium (Suchium) moniliferum (Lamarck), and the thalassinidean ghost shrimp, Callianassa japonica Ortmann, densely inhabited the lower and upper zones, respectively. Callianassa japonica subsequently expanded its distribution range considerably, having occupied almost the entire sandflat by 1983. Concurrently, the U. maniliferum population gradually declined, becoming extinct in 1986. Furthermore, the populations of 9 species associated with U. moniliferum (predators, an ectoparasite, and subsequent inhabitants of empty U. moniliferum shells) had disappeared by the end of 1992. The extinction processes of U. moniliferum and the two other most numerically dominant species, the ectoparasitic gastropod, Odostomia sp., and the hermit crab, Diogenes nitidimanus Terao, were described in detail. It is believed that the bioturbation of sediments by C. japonica was responsible for the extinctions, possible mechanisms involved being discussed. This is the first documented record of the extinction of a large part of a macrobenthic assemblage centering on a filter-feeding Umbonium species which is characteristic of the benthic communities on many intertidal sandflats extending from Japanese to Southeast-Asian waters.