Role of bioturbation and interspecific relationships in determining the distribution of three species of macrobenthos on an extensive intertidal saridflat in Ariake Sound

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Three large-sized macrobenthos dominated the benthic community on an intertidal sandflat with an area of 4.1km^2 located at the mouth of the Shirakawa River in the central Ariake Sound, western Kyushu, Japan. They were the two bivalves (Ruditapes philippinarum and Mactra veneriformis) and the ghost shrimp (Nihonotrypaea japonica) that lives in a deep-reaching burrow. R.philippinarum and M.veneriformis were distributed separately in the lower and middle tidal zones of the 50% of the sandflat, while N.japonica was in the upper zone of the rest of the sandflat. As one possible mechanism that generated such allopatric zonation pattern, two kinds of interspecific competition were postulated ; (1)exploitation competition for food and (2)ammensalistic interference by the ghost shrimp bioturbation of sediments on the bivalves. To demonstrate the above mechanism (1), carbon and nitrogen stable isotope studies were conducted. The diet-tissue isotopic fractionation experimentally measured for the … More three species in the laboratory gave the value of 2‰ for carbon in the ghost shrimp, which was different from the currently accepted values (0-1‰). The application of the fractionation values to the field data has revealed that the food sources for the three species, was restricted to estuarine phytoplankton. The population biomass of the bivalves and the ghost shrimp was estimated at 368 ton and 371 ton, respectively. These results strongly suggest the existence of exploitation competition for food. Concerning the above mechanism (2), a field experiment was conducted, excluding the ghost shrimp from a 2×2m plot with a control plot (8 replicates). Six months later, however, no significant difference in the juvenile bivalve density between the two plots was detected. Under natural conditions, the density of juvenile R.philippinarum was highest in the lowest tidal zone, which was basically followed with bivalve growth for a while. Later adult bivalves had been dispersed as far as the upper part of the lower zone. By contrast, the density of both juvenile and adult M.veneriformis was consistently highest in the middle zone. The stability of the sediment surface was contrastingly different between these tidal zones. It was strongly suggested that substrate stability would be needed for the survival of R.philippinarum and that M.veneriformis would be adapted to unstable substrates. Less