Measuring predation pressures and identification of predators
The modes that predators use to attack gastropods are classified as
shell entry, shell crushing, shell boring, and whole-shell swallowing
(Vermeij, 1979; DeWitt & Robinson, 2000). Some recaptured snails that
were dead showed evidence of shell-crushing, so we considered that they
died from attacks by predators rather than old age. We compared the
number of predated snails between the mainland and the island using
Fisher’s exact test. To examine whether the number of predations was
associated with shell colour and growth stage, we also analysed them
with a generalised linear mixed model (GLMM). Predation (1) and
non-predation (0) were used as binary response variables, while the
shell colour, growth stage (adults 1; juvenile 0), and their interaction
were used as fixed effects. The month when the dead snails were
recaptured was used as a random effect. “lme4” (Bates et al. ,
2007) was used with logit as a link function in the statistical package
R (R Core Team, 2020).
To identify the snails’ predators, camera-trap experiments were
conducted at the mainland study site from December 2019 to July 2020 and
at the island study site from April 2017 to February 2018. At these
study sites, snails were secured to trees with 5-cm-long strings (Daiso
Ltd., Hiroshima, Japan; diameter 0.2 mm) and monitored using trail
cameras (TROPHYCAM HD3 ESSENTIAL, Bushnell Ltd., Kansas, USA). Each
camera was attached to a tree or on a tripod and operated on a motion
trigger system. The camera automatically recorded videos when an animal
entered the camera’s sensor range. 4 and 5 camera traps were set up at
the mainland and island study sites, respectively. The predators were
identified from these videos in our laboratory.