| Research Abstract |
From 1983 to 1994 on the Tomioka Bay sandflat at the northern corner of Amakusa-Shimoshima Island, Kyushu, the population of the ghost shrimp Nihonotrypaea harmandi (Decapoda: Callianassidae) was stable, with high densities of 400 - 1, 300 / m^2, but it has declined since 1995. This was supposed to be caused by increased arrival rates of the stingray Dasyatis akajei, which both disturbs the sediment and ingest shrimps considerably. Its effect on the shrimp population was assessed by following yearly changes in (1) new pit formation on the sediment surface, (2) reduction of shrimps in new pits, (3) stomach content of stingrays, etc. New pits appeared mainly during June to August, with mean densities of 2.3 - 3.4 / m^<-2> ・ d^<-1>. The mean shrimp reduction rates in the new pits were 62 - 78 %. Juvenile shrimps were most affected by the sediment disturbance. The stomach content of the stingray was mainly composed of adult shrimps. It is estimated that 6 to 19% of the shrimp population at the beginning of June was lost during the three months each year. With this rate continued for 6 years, the increased predation/sediment disturbance by the stingray can well explain the observed reduction in the shrimp population. Accompanying such reduction, the populations of the trochid gastropod Umbonium moniliferum and 5 associated species [two hermit crabs, one ectoparasite (gastropod), and two predators (gastropod and turbellarian)], which had went extinct due to the bioturbation by the ghost shrimp, have recovered. These populations had been maintained on two major sandflats along the eastern coast of Amakusa-Shimoshima Island. Apparently, these local populations acted as larval sources for the recovery of the Tomioka populations. The present study has revealed the crucial roles of both the top-down control and the source-sink relationship in metapopulations in restructuring processes for an intertidal sandflat community.
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