潮間帯に生息するコツブムシ科穿孔性等脚類の分布に関係する要因

抄録

The spatial distribution patterns of two species of intertidal boring isopods, Sphaeroma wadai Nunomura and S. sieboldii Dollfus, on the coast of Tanabe Bay, central Japan, were described, and the factors affecting their distributions were investigated. Sphaeroma wadai was distributed mainly in the inner parts of the bay, being found in sandstone and decaying wood. On the contrary, the main distribution areas of S. sieboldii were near the mouth of the bay, and they inhabited sandstone. The hardness of the inhabited sandstone was similar between these two species. Salinity and wave strength (as wave amplitude) at selected stations were measured, and both gradually declined toward the inner parts of the bay. Salinity tolerance tests in the laboratory indicated that S. wadai can better tolerate low salinity than S. sieboldii. Field experiments involving the transplantation of sandstone inhabited by S. wadai to various sites in the bay resulted in the observation that this species had a lower tendency to abandon the transplanted substratum when moved inward in the bay than outward, but the reverse was true in S. sieboldii. Preference tests for wood and sandstone were conducted in the laboratory. Blocks of wood and sandstone were put into aquaria, into which isopods were then released. Both S. wadai and S. sieboldii preferred wood when the wood was soft (hardness=15.45±10.22 kg/cm²), but they chose stone when the wood was hard (20.45±3.84 or 32.94±3.15 kg/cm²). During these experiments, individuals of S. wadai very frequently occurred on the blocks of wood and stone, while more than half the individuals of S. sieboldii were not attracted to any blocks. Observations in tide-simulation tanks revealed that some individuals of S. wadai were active outside of their burrows during nighttime in submerged conditions, while no S. sieboldii were ever active outside of their burrows. This suggests a difference in the intensity of dependence on burrows between the two species. In summary, salinity and wave strength are regarded as the main factors causing differences in the spatial distributions of these two isopod species, although the presence or absence of the substrata they bore into is also considered a primary factor.