Flatworms (Platyhelminthes) are a group of unsegmented worm like animals and as the name suggests they are flattened animals often with a ribbon like appearance. They are found throughout the world, in the UK there are three or four species considered native and ten non-native species that have become established.
All flatworms are predatory with some species feeding on slugs, others feed on earthworms. Australian and New Zealand flatworms are two species that have become established in the UK that feed on earthworms, in some areas earthworm populations have been affected.
The Australian flatworm is orange or pinkish orange and reaches 2-8cm in length. It was first detected in south west England in the 1960s and since then has become more widespread in southern England and Wales. This species does not seem to have had such a big impact as the New Zealand flatworm on earthworm populations.
The New Zealand flatworm reaches 20cm (8in) in length and is dark brown with a paler margin. It became established in Britain during the 1960s and at first it was thought to be of no consequence. It was only when it had become widely distributed, that it was realised that it was feeding exclusively on earthworms, in some places reducing earthworm populations to a very low level. This has undesirable effects on soil structure and also denies earthworms as a food resource for those native animals that feed on them. This flatworm originates from New Zealand and is now thriving, particularly in the wetter parts of Britain, it is most common in Scotland, Northern England and Northern Ireland.
If you find New Zealand flatworms these can be reported to the OPAL New Zealand Flatworm Survey (external link)