American signal crayfish are a species of freshwater crayfish which has slowly done to the indigenous crayfish of Great Britain and much of Europe what the grey squirrel did to the red. It is fast growing and resistant to disease. Native to the rivers of North America.
Freshwater crayfish are crustaceans which resemble small lobsters, with most of the sweet meat in the tail and with red claws almost as large as their bodies. Like lobsters, they are mottled blue-black until they are cooked, when they turn bright red. They breed in autumn (US: fall) and the best are those caught in summer. They seek out unpolluted lakes and streams. Their presence in a dish may be indicated by Nantua on French menus as part of the description. They are sometimes erroneously called crawfish.