Predation impacts of the invasive flatworm Platydemus manokwari on eggs and hatchlings of land snails

Some species of terrestrial planarians (flatworms) are among the predators of land snails, but their predatory impacts have not been sufficiently studied. Flatworms are known to follow snail trails to find prey and enter shells to consume snails; however, eggs (which do not have trails to follow) and snails whose shells are too small for flatworms to enter may not be eaten. To determine whether an invasive flatworm, Platydemus manokwari, preys on snail eggs or small land snails, we conducted a laboratory experiment in which we fed five eggs or five hatchlings (2 mm in diameter) of a common land snail species found in Japan to P. manokwari individuals of various sizes. Egg predation did not occur within 10 days, but hatchling predation commenced on the first night; only 9% of the hatchlings remained on Day 10. Platydemus manokwari did not recognize early-stage eggs as food, but started preying on eggs just before they hatched. Flatworms can therefore be a significant predator on land snails, preying on even tiny land snails, leaving only early-stage eggs free from predation.