Rare leeches find home in dune pools at Newborough

Now we have entered August, we are well into the field monitoring season that sees Natural Resources Wales staff out and about undertaking critical surveys to check on the health of various populations of plants and animals across our protected landscapes.

A recent trip to Newborough National Nature Reserve and Forest on Anglesey saw me doing some invertebrate monitoring of one of our rarest species, the medicinal leech.

The medicinal leech is also one of our most reviled animals. They are of course well known for their feeding on blood and their use in bloodletting in the medical world.

Leeches are typically found in small ponds and lakes, ditches and flooded fen where they feed on the blood of frogs, toads, newts, mammals (especially cattle and horses), fish and the nestlings of aquatic birds.

Tadpoles as well as juvenile newts are especially important for young medicinal leeches unable to pierce mammalian skin for the first two feedings. Incredibly, each leech can take two to five times its own weight of blood in a meal that is digested slowly over several months.

For a large part of the year when water temperatures are low, medicinal leeches are quiescent and remain buried in mud or under submerged objects at the edge of ponds.

But as water temperatures rise in the spring they emerge to feed after a long winter of inactivity. Leeches become very responsive to water disturbance caused by a potential host, and swim towards the source of blood, using heat detection when near a mammalian host.

When feeding they secrete a powerful anti-coagulant enzyme in their saliva called Hirudin which inhibits blood clotting for up to three hours to allow feeding.

Mature medicinal leeches will leave the water in July and August to deposit 10mm long spongy cocoons in a moist place just above the water line on the shore, bank or within the nest of aquatic birds such as coots.

Over one to 12 days, each mature leech will lay one to eight cocoons, each of which contains 12 to 16 eggs. Hatching time varies from four to 10 weeks, depending upon the temperature, and newly hatched leeches measure from eight to 12mm.

The medicinal leech was once widespread in central Europe from Ireland to the Ural Mountains and from southern Scandinavia to countries bordering the Mediterranean.

However, it is now rare throughout western Europe and classed as endangered in many countries.

In the UK, the medicinal leech is restricted to isolated populations in the New Forest, the Lake District, Western Scotland, southern Wales and Ynys Môn, with a large population associated with the extensive ditch system on Romney Marsh in Kent. Historical declines were caused by over-collecting for medical phlebotomy or the bloodletting trade in the 18th and 19th centuries.

Today there are seven sites in Wales with contemporary records for medicinal leeches, of which five are on Ynys Môn and all are protected as Sites of Special Scientific Interest, Special Areas of Conservation or National Nature Reserves.

Most populations are small but one of the largest populations is at Newborough in the various pools that occur in the flower rich grazed clearings.

Many of these clearings were established during the early years of nature conservation at the site in the late 1950s by the Nature Conservancy in partnership with the Forestry Commission.

The then Warden Naturalist for Nature Conservancy, Peter Hope Jones, along with an eminent visiting dune ecologist Dr Derek Ranwell had raised concerns regarding the proposed afforestation and suggested a number of clearings should be preserved to protect the rare flora and fauna and that they should be connected by rides.

Mr Jones went about excavating pools as a mitigation measure and one of these was the dune pool in Pant Mawr, one of the largest slacks on the dune system.

These man-made pools located in light-filled clearings provide a microclimate that is especially favourable for warmth loving invertebrates.

The high pH of the water and clean mineral sand of the pond floor make these some of the best pools of their kind in the UK for specialist endangered species such as rare stoneworts and other aquatic plants as well as populations of dragonflies, Great Crested Newts and of course the medicinal leech.

Threats to the medicinal leech populations today are mainly from loss of wetland habitat as a result of hydrological impacts, succession and climate change, water quality, shading of ponds lowering water temperatures and loss of hosts for feeding such as amphibians and grazing animals.

Recent research has found that dogs bathing in conservation ponds is particularly damaging to the invertebrate interest, including medicinal leech, as flea treatments administered to dogs can enter the water column and kill the insect life.

Please help us protect these rare species by refraining from allowing your dog to use any of these ponds.

NRW will continue to manage these clearings to enhance their condition by removing scrub and continue grazing management with our forest herd of Welsh Mountain ponies.

Image: Pwll Pant Mawr dune pool being excavated in 1959.