Zebra Mussels are thought to have been introduced to Britain in the 1800s on timber imported from the Baltic Sea. They eventually spread through the canal system. Larvae can drift in water for weeks before joining a new population and adults can travel long distances attached to boats. They are prolific breeders, can tolerate low oxygen conditions, and can survive out of water for short periods so are good invaders. They threaten native species by reducing oxygen and nutrients in the water and outcompeting them. There has been one recorded sighting in the River Thame at the confluence with the River Thames.