Medicinal leech  (Hirudo medicinalis)

Curiously, the medical use of leeches might explain their wide distribution across the country, as they are thought to have been released into ponds once they had been used for bleeding patients. It is also thought that over-collecting reduced their numbers in some areas. Over-collecting for medical purposes is unlikely to be a threat these days, given the protected nature of this species and the ability to breed leeches commercially at leech farms. Conversely, the pharmaceutical industry has funded much of the recent work under the UK Species Action Plan for this leech. In Romney Marsh perhaps the most significant issue has been the conversion of grazing marsh to arable cultivation - resulting in lowering of water levels, pollution, and fewer host species. Invasion of scrub around ponds has also been a problem.

The medicinal leech is listed in the UK Biodiversity Action Plans (UK BAP), and is included in English Nature's Species Recovery Programme. As a first step in ensuring its survival as a native species, many of the leech's known sites were designated as Sites of Special Scientific Interest (SSSIs), and a survey was commissioned to establish the species' true status.

An obvious requirement of any species of animal is a food source, and for the medicinal leech this source is other animals. The diet of leeches on Romney Marsh is not perfectly understood. Birds and frogs do seem to be major prey items, but fish have been attacked, and it is suspected that mammals are an under-recorded food item. There are several reports of them feeding on sheep. Scrub clearance around the shallow ponds on Romney Marsh has resulted in leeches taking up residence, and they have also colonised a large number of gravel pits on Dungeness originally created for bird conservation. At the site in Kent, which has the UK's largest leech population and is within the RSPB reserve at Dungeness, water birds provide the food source. At another site, Moccas Park lake in Herefordshire, grazing animals use the lake as a source of drinking water, which suits the leech very well.