Hirudinea - leeches

Class Hirudinea - the leeches

These are the leeches, and are considered the most advanced annelid class. They differ from the other two Classes in 3 important features.

  • The segments at either end have been modified to form attachment suckers around the mouth and another (usually much smaller) at the posterior end. The sucker around the mouth is usually the smaller.
  • Their body functions as a single hydrostatic skeleton as the internal divisions, or septa, have been lost.
  • They do not have Chaetae.

Leech Body plan

Leeches are predominantly freshwater animals, but some are marine and a few terrestrial.

They range in size from 1 - 30 cm in length, but most are between 2 - 5cm long. The longest is the Giant Amazonian leech, Haementeria ghiliana.

Like the oligochaetes they are hermaphroditic, but their clitellum only appears during the breeding season.

Gas exchange occurs through the skin, except for some of the fish leeches with gills. They can have a number of tiny black eyes. They have two brains - one at the head end and the other at the tail end.

Leeches have a fixed number of segments, this is unlike the other Annelid Classes; the number of segments varies with species but is most commonly 17, 31 or 34. They appear to have many more segments because externally each has superficial grooves.

Leech Movement

Leeches have a looping movement, but are also very good and graceful, swimmers, they flatten their body into a ribbon-like shape and propel themselves forward in a series of vertical undulations.

Leech Food

Most leeches are fluid feeders. Blood-sucking leeches drink as much blood as they can from their victims. This blood is stored in special sacs at the side of their intestine and digested later.

Most can consume 3 times their own body weight in one meal. This allows the leech to survive long periods without food.

Freshwater leeches attach to their hosts only long enough to have one blood meal.

Eggs are laid in batches inside a cocoon or capsule attached to a solid object, or carried around on the underside of the adult, depending on species.

Haemopsis sanguisuga, horse leech

Above the horse leech, Haemopsis sanguisuga

Hirudo medicinalis, (the medicinal leech)

Medicinal leech

The Medicinal leech, above, is the biggest freshwater leech in the UK - lengths of 20 cm have been recorded. It used to be fairly widespread but once bridges replaced fords and drinking troughs replaced ponds it became very difficult to find in the wild as it no longer had easy access to its victims.

It has strong teeth which make a Y-shaped cut in the skin, and an anticoagulant in its digestive juices stops the clotting of the victim's blood.

It can consume 2 - 5 times its own body weight in blood, and go without feeding for 18 months. A full meal can take around 200 days to digest.

Medicinal leeches are raised commercially to obtain hirudin - the anti-coagulant used to prevent blood clots. For more details see the table below.

Placoblella sp. (far left) feeds mainly on turtles, but will also feed on alligators if there are no turtles.

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Placobdella sp, leech

Know your leech, a guide to 11 sp. of British freshwater leeches

British freshwater leeches are most easily identified by examining the head shape and eye arrangement, lengths are in mm of fully grown leeches at rest.

Ryncobdellidae - these have a proboscis, colourless blood and no jaw.
Protoclepsis tasselata Olive green, brown, transparent. In large individuals 6 rows of yellow spots down the dorsal surface. 8 in 4 pairs Very active. Feeds on ducks and other water birds entering the nasal cavities and throat. Can lead to death of victim. Egg capsule carried by adult. Young (up to 200 at a time) also carried by adult. up to 60 mm
Protoclepsis tasselata, freshwater leech
Pisicola geometra (common fish leech) Green or brown/red. 8 rows of white spots. Long and narrow. Both suckers large. 2 pairs Very active. Attacks fish. Eggs laid inside brown cocoons 15 mm long and attached to stones etc. 20 - 50 Pisicola geometra
Glossosiphonia complanata Dark green/brown. 2 rows of cream and brown coloured spots running down either side of a dorsal mid-line. Narrow head. 2 or 3 pairs Curls into a ball when disturbed. Feeds on freshwater snails and Chironomid larvae. Eggs are laid an a transparent cocoon attached to a solid object. Found in stagnant pools and running water. 30
Glossosiphonia complanata
Glossosiphonia heteroclita Amber yellow. Dorsal surface may have small black/brown spots. Long oval with narrow head. 3 pairs Feeds on water snails. Up to 60 eggs laid June/July, attached to the ventral surface of the adult. Found in slow, running or stagnant water. 10 - 15
Glossosiphonia heteroclita
Helobdella stagnalis Transparent green/grey/pink. Dorsal surface may have black speckles. Long oval body, narrow head. 1 pair Feeds on freshwater snails, Chironomid larvae, annelids. Eggs carried on ventral surface of parent. Found in ponds and slow moving streams. 10
Helobdella stagnalis
Hemiclepsis marginata (fish leech) Colour varies. 7 yellow spots down dorsal surface. Rounded head. 2 pairs feeds on fish. Eggs attached to ventral surface of adult. 16 - 18
Hemiclepsis marginata
Haemopsis sanguisuga (horse leech) Black/green/olive brown. Both surfaces have black spots. Jaws have 11 - 18 pairs of teeth, but cannot pierce human skin. See drawing above. 5 pairs Feeds on earthworms, snails, insect larvae, tadpoles, dead animals and other horse leeches. Also unlike other leeches it swallows its prey whole. Found in mud in ponds, or on stones in streams. 25 - 35, but can stretch to 150
Haemopsis sanguisuga, horse leech
Hirudo medicinalis (medicinal leech) Olive green with red, yellow, orange and black lines. See the photograph above right. Jaws have many teeth capable of piercing human skin. 5 pairs Cocoons are laid in damp soil. For more details see above right. 30 - 35, but can stretch to 200.
Hirudo medicinalis, medical leech
Herpobdella octoculata Dark brown, also yellow and reddish brown. Black marks. Body long and narrow. 4 pairs Eats small worms such as Tubifex, and planarians. Eggs laid in dark brown transparent cocoons attached at both ends to a solid object. Found in running and stagnant water. 30 - 40
Herpobdella octoculata
Herpobdella atomaria Green/brown with a paler ventral surface. Red/yellow spots and a black pattern. Resembles H. octoculata. 4 pairs Food as H. octoculata. Egg cocoons brown transparent and attached to solid objects. Found in running and stagnant water.  
Herpobdella atomaria
Trocheta subviridis (amphibious leech) Grey/green or reddish with paler ventral surface. 2 brown dorsal lines. Long, worm-like shape. 4 pairs Feeds on earthworms and insect larvae. Will leave water to search for food. Lays flat, brown elliptical egg capsules attached to solid objects. 80 - 100
Trocheta subviridis
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