Medicinal leech  (Hirudo medicinalis)

The medicinal leech has been recorded in 24 countries throughout Europe as far as the Ural mountain range. It is scarce in France and Belgium, and is thought to occur in more than 20 scattered populations in the UK. These extend from Kent to Argyll and Islay in Scotland, and from Norfolk to Anglesey.

The leech is an amphibious freshwater animal, and usually found in small pools with muddy bottoms and fringed with reeds. These pools are also shared with frogs, toads or newts for at least some of the year. Leeches prefer water that is eutrophic, meaning it is high in nutrients. Where amphibians are not present in leech infested ponds, it is believed that grazing stock provide the food source.

On Romney Marsh in Kent, there are extensive populations of the medicinal leech living in the shallow parts of deep gravel pits, and in ditches that run through the grazing marshes. In Cumbria, leeches have been found in conditions that differ markedly from those in Kent. Here, it is thought that being shallower, the water warms up quickly to about 20°C, a temperature that must be attained for at least part of the year in order for leeches to survive in a pond.