Potential impacts of the invasive flatworm Platydemus manokwari on arboreal snails

The introduction of the snail-eating flatworm Platydemus manokwari (Tricladida: Rhynchodemidae) has been considered a cause of the extinction of native land snails on several Pacific islands. Although P. manokwari is known to attack land snails on the ground, whether P. manokwari attacks snails on trees remains unclear. To clarify the effect of P. manokwari on arboreal snails, we examined survival rates of land snails experimentally placed on tree trunks (0.5–2.0 m above the ground) in a forest on Chichijima, Ogasawara (Bonin) Islands, in the northwestern Pacific Ocean. The survival of snails experimentally placed on tree trunks with artificially created snail scent trails rapidly decreased for 7 days, and the mortality was caused by P. manokwari predation. However, snails placed on tree trunks without snail scent trails were not attacked by P. manokwari. Therefore, P. manokwari climbed tree trunks, likely tracking the snail scent. We found that over 40% of the snails placed on tree trunks with snail scent trails were eaten by P. manokwari within 7 days. This experiment supports the hypothesis that P. manokwari predation is an important cause of the rapid decline or extinction of native arboreal snails on Pacific islands.