Data collection

The Apple Snail Research Program was conducted from June 2017 to December 2019. The program was announced to the members of the Friends of the Osaka Museum of Natural History (members included approximately 1600 families). In addition, I used twitter (screen name: @soishida) to publicly announce the program. I asked people to send the following data when they found the egg capsules: 1) observation date, 2) locality (place name and/or geographic coordinates), and 3) photograph of egg capsules. Data transmission was also accepted through tweets with a specific hashtag (#スクミ調査). An identification training in the field and two indoor seminars were conducted for the members who wanted to participate in the 2017–2019 ASRP program (Figure 3). Photographs were optional when the volunteers were confident of identification. Identifications were correct in all of the reports that appended photographs (approximately 380 reports). I also accepted reports of finding the snail shells. I confirmed with the volunteers directly for identification accuracy for a few reports as and when the need arose. In addition, absence data were collected. If volunteers searched for eggs or snail shells within a 500 m × 500 m area for 30 min and were not able to find them, they reported the geographic coordinates of the center of the area as an absent plot. I recommended volunteers to get the geographic coordinates (latitude and longitude) by using Google Maps (https://www.google.co.jp/maps/) or Geospatial Information Authority of Japan’s website (http://maps.gsi.go.jp/). When they reported only the place name (e.g. address with lot number, or landmark), I converted it into coordinates by searching on Google Maps. All of the coordinate values in this dataset were recorded to at least three decimal places in degrees (most of them hold six decimal places) and conform to the world geodetic system (WGS84 or ITRF). During the program, I started a website (https://sites.google.com/site/sukumiringo/) to explain the method and display the progress of the research. The reported data were plotted on the map system embedded in the website. Updates were made at least once every few weeks. One of the main purposes of this system was to inform volunteers in the unexplored areas. When I received data, I asked all volunteers for approval to display their names with their respective reported locations. Names of volunteers who wanted to be anonymous were masked or replaced with a nickname in the data. For volunteers who sent the reports by tweet, their twitter screen names were used as their names. The policy of open or masked names was applied to this dataset as well. The Apple Snail Research Program did not aim to distinguish Pomacea canaliculata, P. maculata and their hybrids. There are statistically significant differences in egg and clutch size among them (Hayes et al., 2012; Matsukura et al., 2013); however, it would be difficult to identify them by using only the external appearance of the eggs as a key character. During this program, I received 1820 reports. Two reports were received from Taiwan and Korea (recordNumber 347 and 1820, respectively), and some volunteers reported several observations before the program period (6 cases in 2014–2016, recordNumber 43, 44, 45, 65, 128, and 174) which were excluded from this dataset. In total, 1812 records (1304 presence and 508 absence) were prepared.