Forget Nessie: there's another insidious creature living in the waters of Scotland.
The story starts in the streams and lakes of the northwestern United States, where North American signal crayfish are a familiar sight. Turn over a rock and you may well encounter one.
But in Scottish streams and lochs, these creatures are intruders.
In the United States, we often hear about invasive Asian carp, zebra mussels or snakehead fish from China that take over American waterways. It's a two-way street: American species are causing chaos in other parts of the world, too.