北海道東部厚岸湖におけるマルテンスマツムシMitrella martensi（腹足目：Columbellidae科）の出現報告および個体数と植物量の関連に関する予備的調査

抄録

A columbellid gastropod, Mitrella martensi (Lischke, 1871), known from the temperate Asian intertidal coasts, is now considered endangered in Japan. In this study, we have reported its occurrence in the seagrass meadow of Akkeshi-ko Estuary, Hokkaido, northeastern Japan, as the northernmost record of this species. The observed population was considered part of the native Japanese population based on historical occurrence records from Akkeshi-ko Estuary and northern Honshu Island during the early 1900s, as evidenced by museum specimens and the literature, together with a morphological examination. M. martensi occurred in the seagrass bed and at an oyster farm adjacent to the bed. However, individuals with mature shells were only found at the oyster farm. The field survey in seagrass beds and a generalized linear modeling approach suggested a significant but marginal association between the abundance of M. martensi and the algal dry weight, but not with the seagrass density or the seagrass dry weight. This result is consistent with a previous study suggesting this species inhabits tidal flats and seagrass or algal beds with pebbles. Macroalgae attached to cobbles and pebbles were associated with M. martensi abundance. Meanwhile, seagrass with belowground roots was not. The findings show that M. martensi is distributed in subarctic northeastern Japan. Hard substrates, such as pebbles, oyster shells, and artificial substrates, in seagrass beds may play a crucial role for the species.