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

在日本九州西部有明湾中部白川河口的一个面积为4.1km^2的潮间带滩涂上，3种大型底栖动物主导了底栖生物群落。它们是两种双壳类（菲律宾Ruditapes philippinarum和Mactra veneriformis）和生活在深海洞穴中的鬼虾（Nihonotrypaea japonica）。在50%的滩涂滩涂中、下潮区分别分布着菲律宾沙虫和veneriformis，而日本沙虫则分布在其余滩涂的上潮区。作为产生这种异域带格局的一种可能机制，我们假设了两种种间竞争；(1)对食物的开发竞争；(2)双壳类动物上沉积物的鬼虾生物扰动对双壳类生物的干扰。为了证明上述机理(1)，进行了碳、氮稳定同位素研究。实验测定的3种虾的饮食-组织同位素分馏值为2‰，与目前公认的0-1‰不同。分馏值对野外资料的应用表明，这3个物种的食物来源仅限于河口浮游植物。双壳类和鬼虾的种群生物量分别为368吨和371吨。这些结果有力地表明存在着对食物的剥削竞争。针对上述机理(2)，进行了现场试验，将鬼虾排除在2×2m小区和对照小区（8个重复）中。6个月后，两样地双壳贝幼鱼密度无显著差异。在自然条件下，菲律宾r.p inarum幼鱼密度在最低潮区最高，随后一段时间基本为双壳类生长。后来，成年双壳类已分散到较低地带的上部。在中部地区，幼虫和成虫的密度均最高。在这些潮带之间，沉积物表面的稳定性有很大的不同。研究结果强烈表明，菲律宾芽孢杆菌的生存需要稳定的底物，而veneriformis可能适应不稳定的底物。少

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