北限のスナガニの季節的な砂浜利用パターン：啄木はスナガニに出会えたか？

抄録

The occurrence of the ghost crab Ocypode stimpsoni was surveyed at 32 beaches within southern Hokkaido, and Itanki Beach at Muroran was detected as the northernmost edge of its range. The burrow distribution of the ghost crab at Omori Beach, Hakodate, near the northern edge, was investigated from January to December 2014. Burrow opening was initially observed in late June near the upper area of the beach with vegetation. The number of burrows increased in the following months, with their distribution extending down the beach. During mid-summer with high temperature, severe dryness of the sand restricted the upper limit of the burrow digging area to immediately below the high-tide water mark with moist sand. The lower limit was determined by excavation of the sand by rough waves, concentrating the burrow area to a narrow band within the beach. In autumn, the dryness decreased and the burrow area extended upward, becoming wider. Burrow activity halted by the end of October. After October, the ghost crab overwintered in deep burrows within the upper area until late next June. These results suggest that at the northernmost edge of the distributional range, seasonal activity patterns are severely restricted by environmental factors such as extreme temperatures, dryness, and rough waves, which is coincident with the general rule of geographical range pattern of organisms. A possibility that the famous poet Takuboku Ishikawa played with the ghost crab at Omori Beach was discussed based on the findings.