SD: Standard Deviation

Figure 1. A) Maps of study sites in the Izu Peninsula.Euhadra peliomphala simodae is distributed on the Izu Peninsula (mainland) and the Izu Islands including Niijima Island. The survey was conducted on both the Izu Peninsula and Niijima Island. B, C) The cross-section of the Izu Peninsula and Niijima Island from the west side. In the Izu peninsula, E. p. simodae is inhabited only in southern part of it.

Figure 2. Frequency shell colour distribution on marked snails in A) the Izu Peninsula and B) Niijima Island (modified from Ito and Konuma, 2020). Shell colour was quantified using the luminance value\(Y\) using a photograph of the snail, with higher values representing brighter colours. In the Izu Peninsula, the distribution showed unimodal and mainly bright shell colour existed whereas it showed bimodal, dark and bright shell colour existed in Niijima Island.

Figure 3. Images of predated empty shell and the large Japanese field mouse Apodemus speciosus . A) the scene that A. speciosus predated a living snail which was tethered using sting in front of a trail camera. B) the shell left after a trail camera captures a scene that A. speciosus preyed on a snail. C) empty shells predated by A. speciosus found in mark-recapture survey. It has characteristics the splitting of the apex or the shattering of the shell.

Figure 4. The relationship between survival rate and shell colour in A) the Izu Peninsula and B) Niijima Island (modified from Ito and Konuma, 2020); I) is juvenile, II) is adult. This shape represents the natural selection worked on shell colour. In the Izu Peninsula, the natural selection was estimated after excluding predation effects. The solid and dashed lines indicate the median and the 95% Bayesian confidence interval, respectively, in the posterior distribution obtained from Markov chain Monte Carlo simulations.