Temporal changes in molluscan community structures over 10-year and 13-year periods before and after the 2011 Great East Japan Earthquake and subsequent tsunami around the Tona Coast, Matsushima Bay, Miyagi Prefecture, northeastern Japan

We examined structural changes in the molluscan community around Matsushima Bay, Miyagi Prefecture, northeastern Japan, for 10 years (2001–2010) before and 13 years (2011–2023) after the 2011 Great East Japan Earthquake and the subsequent tsunami. Before the earthquake and tsunami, Ruditapes philippinarum, Macoma incongrua, Pillucina pisidium, and Batillaria attramentaria were dominant, whereas the alien species Laguncula pulchella appeared in 2002 and increased in number. After the tsunami, R. philippinarum and M. incongrua populations quickly recovered in 2012; however, the intertidal snail B. attramentaria population did not start to recover until 2020. In contrast, Arcuatula senhousia, Mya japonica, Decorifer matusimanus, Solen strictus, and Cylichnatys yamakawai were not recorded or were found at low densities before the tsunami; however, their abundance rapidly increased after the tsunami. These results suggest that the molluscan community on the Tona Coast was drastically altered by the earthquake and tsunami. Some species mainly inhabiting the intertidal–subtidal zone may have increased in number because of land subsidence and sediment scouring near the seawall caused by the tsunami, which expanded the subtidal habitat. Although the molluscan community on the Tona Coast is approaching its original state, the recovery is slower than that of other tidal flats because of the decrease in intertidal habitat. Without restoration of the intertidal habitat, the molluscan community is likely to enter another state as a mixture of characteristic species, before and after the earthquake and tsunami.