The present study was conducted to elucidate the distribution of the invasive alien snail Laguncula pulchella and its effect on the native mollusk populations in the Orikasa River Estuary, Iwate Prefecture after the 2011 Great East Japan Earthquake. Laguncula pulchella was the most abundantly distributed in the area with sandy sediment among the three survey areas. Although the number of L. pulchella increased in an area with gravelly sediment during the second year of the study period, the higher gravel content (>54%) might have inhibited the distribution of L. pulchella in another gravelly area. The clam Ruditapes philippinarum was the dominant species and main prey of L. pulchella in the study area. There was a positive correlation between predator and prey sizes, and the coefficient (0.53) tended to be lower than that reported by the earlier studies. In the estuary, L. pulchella might have preyed on clams smaller than its preferred size. The density of R. philippinarum decreased within the distribution area of L. pulchella; however, its density was maintained in the areas with gravelly sediment. These results indicate that the heterogeneity of sediment with gravel after the earthquake limited the distribution of L. pulchella, and thereby contributed to the maintenance of the native mollusk populations.
