The Northern River Crangonyctid is a bluish white shrimp around 4-7mm long. It often occupies highly polluted waterways where no other shrimp can survive. It is native to North America but was introduced to England in the 1930s and has spread since then. They can be highly reproductive even in poor and changing environmental conditions, and have the ability to survive out of water for short periods, so have been very successful invaders. They have been found mainly in the lower catchment but also in the Grand Union Canal in Aylesbury.