科研之友
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摘要
Previous research has shown that on an extensive intertidal sandflat in Ariake Sound, Kyushu, Japan, the populations of two powerful bioturbating shrimp species, Upogebia major and Nihonottypaea japonica, formerly predominated over the high to mid-tide zone but considerably declined almost to the extirpation during 2004 to 2008. This event was followed by higher recruitment of three infaunal clam species, Ruditapes philippinarum, Meretrix lusoria and Mactra venenformis, than before, most probably owing to competitive release from the shrimp. Thereafter, Me. lusoria and Ma. venenformis were successful in establishing their adult populations in the newly vacated habitat, but R. philippinarum failed. The surface sediment is more easily eroded by hydrodynamic forcing there than in the low-tide zone where the R. philippinarum population persisted. In the present study, the examination of spatiotemporal size-frequency variations in the clam species detected that their success/failure was primarily caused by the survival of juveniles in the first winter after recruitment. The assessment of potential sediment erodibility through time suggested that the seabed of the high to mid-tide zone was subject to more intense physical disturbance in winter than in the other seasons, especially during the nighttime. This was attributable to seasonally varying (1) prevailing wind directions, (2) daily nighttime length and (3) emergence/submergence durations per tide according to the seasonal change in diurnal tidal-height inequality. We hypothesized that some high burrowing ability of juvenile clams is required for their persistence in the unstable sediment, which was tested by laboratory and field experiments. The burrowing performance of juveniles was higher in Me. lusoria and Ma. venenformis than in R. philippinarum and was reduced under dark against light conditions in all species. For those juveniles occurring in the high to mid-tide zone, vulnerability to sediment erosion would increase at nights in the wintertime through a combined effect of the stronger wave action and the extension of both nighttime length and nighttime duration of submergence with very shallow waters. With forced repetitive re-burrowing, R. philippinarum reduced burrowing performance, whereas the other two species either enhanced or maintained it. Thus the interspecific difference in the resistivity of juvenile clams to unstable sediments can explain their overwintering success/failure in the high to mid-tide zone. The present findings provide a new perspective for the mechanism of winter mortality of infaunal bivalves inhabiting tidal flats in temperate to boreal regions.
- 出版日期2015-5
