Fascinating Invertebrates ~ Bring on the leeches!

Saturday 22nd April 2023


To celebrate Earth Day (22 April) and our current Saving Scotland’s Special Invertebrates Green Match Fund campaign, Buglife Conservation Director, Craig Macadam, takes us on a deep dive into the world of a much maligned and misunderstood species.  Leeches.  Join us a we shine a light on these intriguing creatures…

Despite their gruesome reputation leeches are fascinating animals.  Their populations are dependent on the availability of suitable hosts which makes them good indicators of ecological conditions in ta waterbody.  They are found in almost all fresh water habitats, including ponds, ditches, lakes, wetlands, springs, streams and rivers, and the Trocheta spp. are often found hunting earthworms on adjacent land.

Leeches are epimorphic, meaning that their form remains the same throughout their development.  British and Irish leeches range in size at rest from 7 millimetres in Piscicola siddalli, up to 160 millimetres in Trocheta subviridis.  When fully extended the larger leeches can be over 200 millimetres long.

The leeches body is split in to 34 segments (2 head segments and 32 body segments) which are further split into a varying number of annuli, ring-like structures on the surface of the segments.  On the segments there may be prominent raised processes called papillae.  Whilst many leeches are quite plain, some have distinctive patterns of stripes and spots, and can be quite colourful.

There are suckers at both the head and rear of the body.  The rear sucker is usually fairly obvious, however the sucker at the head end may be less well defined.  The suckers are used for movement, with the rear and head suckers attached to a surface alternately creating a distinctive looping action.

On the dorsal surface (top) of the head there are between one and five pairs of eyes.  The arrangement and number of eyes is important for identification.  Occasionally individual eyes may be missing however the arrangement can usually be worked out from the remaining eyes.

All British leeches are hermaphrodites, however, they can also reproduce sexually – with some species having a breeding season.  The time between mating and egg-laying varies between species.

Young leeches develop in eggs, which are enclosed inside cocoons.  The number of eggs inside an individual cocoon varies between species.  Leech cocoons are fascinating structures, and they can be used to identify the parent leech at least to family level.  Cocoons are mainly laid on objects either in the water or just above the water line.  However, one family of leeches, the Glossiphoniidae, brood their fertilised eggs and carry their young on their ventral surface (underneath).

The developmental time of eggs inside cocoons varies depending on both species and environmental conditions.

Leech © Anne Burgess CC BY-SA 2.0
Leech © Anne Burgess CC BY-SA 2.0

Leeches feed on a range of prey depending on the species.  The Glossiphoniidae mostly feed on other invertebrates, particularly snails and have a proboscis which they insert into the body of the prey and suck out the body fluids.  There are three exceptions to this general behaviour.  Hemiclepsis marginata may feed on fish and amphibian tadpoles; Theromyzon tessulatum specialises on feeding in the nasal cavity of ducks; and Placobdella costata feeds on adult amphibians, water birds, and mammals, including occasionally humans.

The Erpobdellidae typically feed by swallowing their prey of small invertebrates such as non-biting midges whole.  They may also suck the bodily fluids from invertebrates, and they will also scavenge on dead and decaying fish and amphibians.  The Trocheta species will often leave the water and move across land in search of their earthworm prey.

The Piscicolidae feed exclusively on fish.  Like the Glossiphoniidae, they have a proboscis which they use to penetrate the skin on the underside of the fish.

The Haemopidae and Hirudidae both have jaws however these are rather weak in the Horse Leech (Haemopis sanguisuga), which feeds by swallowing small invertebrates and amphibians whole or on the bodies of wounded or dead fish and amphibians or other carrion.  Contrary to its name, the jaws of the Horse Leech cannot pierce the skin of mammals.  ‘Horse’ in this context is thought to be from the Anglo-Saxon word for ‘false’, referring to their ineffectiveness in blood-letting.

Medicinal Leech (Hirudo medicinalis) © Neil Phillips
Medicinal Leech (Hirudo medicinalis) © Neil Phillips

The feeding behaviour of the Medicinal Leech (Hirudo medicinalis) perhaps makes it the most notorious of the British species.  This species has three strong jaws which it uses to pierce the skin of wading mammals such as cattle, sheep, deer and humans.  Its saliva contains a powerful anti-coagulant which keeps the blood flowing by preventing clotting.  In addition to mammals, the Medicinal Leech will also feed on fish and birds such as Coots and Moorhens when they are on their nests, and on amphibians, particularly in their breeding season.

Blood-letting with leeches was first reported in Europe in the 16th century, although it is thought that their use dates back to around 130BC in Greece.  It wasn’t until the late 18th / early 19th century that leeches became a commonplace medical treatment.  At its peak, an estimated 5 to 6 million Medicinal leeches were used in Parisian hospitals in the first half of the 19th century.  The demand for leeches led to their export around Europe and even to North America. Leech-collectors, often women, were employed to gather leeches from wild habitats, often by wading bare-legged in the water and allowing the leeches to bite them.  This exploitation lead to the decline of populations across Europe.  In Ireland the Medicinal Leech has been extinct since the late 19th century, and in Britain it was thought extinct until a small number of populations were rediscovered in the latter half of the 20th century.

Buglife is leading exciting work to help the Medicinal Leech in Scotland.  There are only two populations of this species left in Scotland.  As part of the Species on the Edge programme Buglife is working with the Kildalton Estate on Islay to stabilise water levels in the lochan where the leeches are found; in addition removal of invasive Rhododendron from the immediate area surrounding the lochan is also taking place.

We’re also working with the Royal Zoological Society of Scotland to establish a captive breeding programme for Medicinal Leech in Scotland with a view to introducing them in to other lochs on Islay.

Thanks to the Big Give 𝐘𝐎𝐔𝐑 donations to Buglife were 𝐃𝐎𝐔𝐁𝐋𝐄𝐃 from midday 20-27 April 2023, together we raised a huge £38,984 for Scottish invertebrates.

Main Image Credit: Medicinal Leech (Hirudo medicinalis) © Neil Phillips