Sometime before 1995, a container of freshwater crayfish from Florida got too hot or too cold en route to a pet shop in Germany. The shock disrupted the development of an egg being carried by one of the females, creating an army of clones that are invading rivers and lakes in continental Europe , Madagascar and Japan.

That, at least, is our best guess of what happened. And it has now been shown that this self-cloning crayfish, known as marbled crayfish, is so distinct from the …