Mark-recapture experiments
We chose the Izu Peninsula and Niijima Island as sites to study E. p. simodae populations on the mainland and an island, respectively (Figure 1). According to a previous study (Hayashi & Chiba, 2004),E. p. simodae that was distributed in the southern part of the Izu Peninsula migrated to the Izu Islands including Niijima Island, and then their shell colour diversified. A snail population in Minamiizu-cho on the Izu Peninsula was examined as a mainland ancestral population. A population on Mount Miyatsuka on Niijima Island was examined as an island descendent population. Both sites were narrow forest areas that were about 1 km long.
Mark-recapture surveys were conducted every month on the mainland (February 2019 - February 2020) and the island (March 2017 - October 2018). The details of the mark-recapture approach are described in a previous report (Ito & Konuma, 2020), so we include only an overview here. Each survey was conducted for 2–5 days. When we captured snails, we recorded their growth stage, geographic locations, and shell colour. The growth stage (juvenile or adult) was identified based on the shell lip. The locations where we captured snails were recorded using mobile GPS devices (eTrex LEGEND HCx, Garmin Ltd., Schaffhausen, Switzerland, on the mainland and a Garmin eTrex 10J on the island).
The shell colour was quantified using RGB values of photographs. Photographs of the snail shells were taken using a digital single-lens reflex camera (EOS Kiss X7; Canon, Tokyo, Japan) with an EF-S 60-mm f2.8 USM macro lens (Canon) under a ring light in a light-blocking box. On a computer monitor, a square area of a shell close to the shell aperture was cropped using the software Photos for Mac OS X (Apple Inc., Cupertino, CA, USA). 1000 pixels within cropped images were randomly chosen, and the RGB values of those pixels were automatically measured and averaged using Python 3 code.
The luminance value \(Y\) of the shell colour was calculated as follows:\(Y\ =\ 0.299R\ +\ 0.587G\ +\ 0.114B\) (Schlick, 1995). The luminance value \(Y^{\prime}\) of the grey scale image (Kodak Color Grey Scale; Eastman Kodak Co., Rochester, NY, USA) photographed with the shell together was also measured in the same way. \(Y/Y^{\prime}\) was calculated to correct minor differences caused by light conditions, and then they were standardized. A higher \(Y/Y^{\prime}\) value implies a brighter shell colour, whereas a lower \(Y/Y^{\prime}\) value implies a darker shell colour.
The captured snails were marked on the shell by gluing on coloured plastic beads and scarifying numbers on them (Pierre-Yves Henrya & Jarne, 2007). They were then released at the locations where they were caught. This study was conducted according to the animal experimentation regulations of Tohoku University and Toho University.