Occurrence and distribution of early juvenile land hermit crabs at a small beach on the Boso Peninsula, Japan

Abstract The present study was conducted to examine the occurrence and distribution of early juvenile land hermit crabs of the genus Coenobita on a small beach on the Boso Peninsula, Japan as a basis for understanding the important habitats that land hermit crabs use during the sea-to-land transition. The survey sites were supratidal zones at six points with heterogeneous topography, and visual surveys for early juveniles were conducted from middle July to late October in 2015 and 2016. The number of empty gastropod shells in four quadrats along the intertidal zone at each point was also counted in the 2016 survey. The occurrence of early juveniles varied among survey points and was spatially stable for two successive years. The first appearance and abundance of early juveniles varied among survey years. Early juveniles were abundant at points with a gravel bottom throughout the intertidal zone, and empty gastropod shells were also abundant at these points. The abundance of early juveniles was highly correlated with the amount of empty gastropod shells. Thus, our surveys suggested the importance of the sediment types and abundance of empty gastropod shells in the intertidal zones for settlement and landing by land hermit crabs.

摘要 本研究旨在调查日本房总半岛一处小型海滩上陆寄居蟹属（Coenobita）早期幼体的出现情况与分布规律，以期为理解陆寄居蟹从海洋向陆地过渡阶段所依赖的关键栖息生境提供依据。本次调查在6个地形异质性不同的潮上带（supratidal zone）点位设置采样区域，于2015年7月中旬至10月下旬、2016年同期开展早期幼体的目视调查。2016年的调查中，还对每个采样点沿潮间带（intertidal zone）设置的4个样方（quadrat）内的空腹足类（gastropod）贝壳数量进行了计数。早期幼体的出现情况因采样点而异，且在连续两年间保持空间稳定性；但其首次出现时间与种群丰度则随调查年份有所变化。在潮间带全程以砾石为底质的采样点中，早期幼体种群丰度较高，且此类点位的空腹足类贝壳丰度同样较为丰富。早期幼体的种群丰度与空腹足类贝壳的数量呈高度相关。综上，本调查结果表明，潮间带的沉积物类型与空腹足类贝壳丰度，是陆寄居蟹定居与登陆过程中的关键影响因素。