The most widely publicised native animal overseas probably got to Britain hidden in potted plants. This is the planarian flatworm Arthurdendyus triangulatus. Planarians are common in New Zealand – these slow-moving black flatworms are often found under rocks or flower pots left out in the garden. Britain, by contrast, has only a few native planarians.
The flatworm made tabloid headlines in Britain when it was discovered that this slimy ‘alien’ had invaded their green and pleasant fields and was eating its way through the local earthworms. Several other New Zealand planarians have followed Arthurdendyus triangulatus to Britain, and there are now as many New Zealand species there as native British species.
Another successful New Zealander abroad is a small freshwater snail, Potamopyrgus antipodarum. It is common in streams and drainage ditches in New Zealand, but when it was found in London’s River Thames in the 1850s, British scientists did not recognise it. They assumed it was a rare local species and gave it a new name.
The snails probably got there in sailing ships’ water barrels filled from streams in New Zealand. They are adapted to brackish water, so could live in the barrels until they were eventually tipped out with the last foul dregs into the Thames after the long voyage from the Antipodes.
During the 19th century the snail spread through the streams and rivers of Britain and then across to the continent of Europe – and in each country scientists gave this unknown species a new name. Suggestions that it might be similar to a species known from New Zealand were ignored until in the 1970s two New Zealand scientists showed that it was in fact Potamopyrgus antipodarum. The ‘New Zealand mudsnail’, as it has become known, is continuing to spread through the streams and rivers of Europe and now the United States, displacing local snails.