First record of the New Guinea flatworm (Platyhelminthes, Geoplanidae) as an alien species in Hong Kong Island, China.

The New Guinea flatworm () caused extinction of the native land snails on several Pacific island in past decades, and therefore it has been listed among the top 100 of the world's worst invasive alien species. Using morphological and molecular methods, New Guinea flatworms were discovered and identified for the first time in Hong Kong Island during a field investigation in July and August 2018. The flatworms were 32-60 mm long, 3-5 mm wide, and 1-2 mm thick. The dorsal side of the flatworm was dark brown with a thin yellow central line, and its ventral side appeared pale grey. To further verify this species, both 18S rDNA and mitochondrial cytochrome c oxidase subunit I gene (COX1) obtained from three specimens of were sequenced and analysed. While comparing these sequences with those previously deposited in GenBank, these 18S rDNA sequences shared 100% identity with the single available 18S rDNA sequence of ; and the obtained COX1 sequences were identical to those of world genotype. Two native snails, and , have been found to be the prey of this predator during this investigation. Therefore, the invasive New Guinea flatworm certainly will cause a serious impact on the biodiversity of native snail populations, and an economic and environmental risk assessment for need to be completed in the near future in Hong Kong.