Signal Crayfish are lobster-like animals of around 16-18cm. They are native to western North America and were originally transported to Britain to be farmed as a food source. Individuals escaped and the population quickly expanded and spread. They can disperse widely as they can move easily up and down stream as well as across land. They are causing problems by outcompeting native crayfish and spreading crayfish plague. They also feed on many native species, altering food webs. They have been found catchment-wide in the River Thame.