Prey preference and gregarious attacks by the invasive flatworm Platydemus manokwari

The flatworm Platydemus manokwari (Tricladida: Rhynchodemidae) preys on various species of land snail, and its introduction to areas outside of its native range is thought to have caused the extinction of native land snails on several Pacific islands. Platydemus manokwari occurs in areas where land snails have been absent since its invasion, suggesting that the flatworm can prey on animals other than land snails. To identify the alternative prey and prey preferences of P. manokwari, I examined the feeding ecology of P. manokwari under field and laboratory conditions. Individuals were observed attacking live earthworms in a forest where land snails are already extinct, on Chichijima, Ogasawara (Bonin) Islands, Japan. I also observed them attacking earthworms and a species of isopod and land snail in the lab. To prey on the worm, similar to other earthworm-eating flatworms, P. manokwari wrapped itself around the prey and fed on it by inserting its pharynx into the earthworm’s body. Large earthworms were able to escape P. manokwari attacks by autotomy, but the autotomized body parts were eaten. Several P. manokwari individuals together attacked earthworms that were larger than themselves. The laboratory experiments showed that such gregarious attacks increased predation success on both large earthworms and land snails. The flatworms also attacked the isopods, although the predation rate was low. These results demonstrate that P. manokwari is a polyphagous predator of slowmoving soil invertebrates (land snails, slugs, earthworms, and isopods) and that invasion by P. manokwari may directly and indirectly impact native soil fauna.