Hirudo medicinalis

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Geographic Range

The range extends through parts of western and southern Europe to the Ural mountains and the countries bordering the northeastern Mediterranean (Sawyer, 1986).

Habitat

The medicinal leech is amphibious, needing both land and water, and resides exclusively in fresh water. A typical habitat for H. medicinalis would be a small pond with a muddy bottom edged with reeds and in which frogs are at least seasonally abundant (Sawyer, 1986).

  • Aquatic Biomes
  • lakes and ponds

Physical Description

The medicinal leech has a cylindrical, dorsoventrally flattened body divided into thirty-three or thirty-four segments. The dorsal side is dark brown to black, bearing six longitudinal, reddish or brown stripes, and the ventral surface is speckled. All members bear a posterior and anterior disk-shaped sucker. The anterior sucker surrounds the oral opening where the teeth for incison are located. In addition, the medicinal leech has five pairs of eyes located on its front end. H. medicinalis has several pairs of testes and one pair of ovaries as well as a thickening of the body ring, known as a clitellum, which is visible during the breeding season (Grzimek, 1974).

Reproduction

H. medicinalis breeds once during an annual season that spans June through August. It also remains fertile over a period of years,unlike most other leech species. The act of copulation takes place on land, where one leech attaches ventrally to one another by means of a mucus secretion. All leeches are hermaphroditic and fertilization is internal. Sperm is injected into the vagina by an extendable copulatory organ. A cocoon is formed around the clitellum and slips off the anterior section of the leech. The whole egg sac is laid in damp soil usually just above the shoreline. After about 14 days, the eggs hatch as fully formed miniature adults (Grzimek, 1974; Sawyer 1986).

Behavior

Motility is achieved both in land and water. Hirudo medicinalis moves in water by contraction of the longitudinal muscles of the body in a wave-like motion which propels it forward in the water. Movement on land is accomplished by means of "looping", a movement similar to that of inch worms. They attach themselves to the substrate alternately by their anterior and posterior suckers.

Resting posture

While at rest, the medicinal leech lies under large objects on the shoreline, partially out of water.

Shadow reflex

The leech is able to detect the movement of shadows above. Especially when "hungry" H. medicinalis will respond to moving shadows, which often indiciate a source of mammalian food. A dark shadow may also set off an alarm response in the leech in which it halts ventilation.

Sensory behavior

The leech is sensitive to light, heat and dessication. It becomes desensitized during feeding and copulation to the point where its posterior end can be cut off and it will continue the same behavior (Grzimek, 1974; Sawyer 1986).

Food Habits

Hirudo medicinalis is parasitic and the adults feed on the blood of mammals. It attaches to the host by means of its two suckers and bites through the skin of its victim. Simultaneously, the leech injects an anaesthetic so that its presence is not detected, and an anticoagulant in order for the incision to remain open during the meal. It has three jaws, which work back and forth during the feeding process, which ususally lasts about 20 to 40 minutes and leaves a tripartite star-shaped scar on the host. After a full meal of 10ml to 15 ml of blood, the medicinal leech may increase 8 to 11 times its initial body size. Leeches only feed about once every six months, this is about how long the blood meal takes to be fully digested. Certain bacteria keep the blood from decaying during the long digestion period. H. medicinalis may even go longer than six months without food by digesting its own tissues.

Young leeches feed on frogs instead of mammals because their jaws are not yet strong enough to cut through mammalian skin (Grzimek, 1974; Sawyer, 1986).

Economic Importance for Humans: Positive

The medicinal leech, as its name suggests, has historically been used for medicinal purposes, mainly to remove "bad blood" from the diseased. Around 1850 this practice fell into disrepute, but H. medicinalis is again becoming of value in medicinal practices. Today this species is used to relieve pressure and restore circulation in tissue grafts where blood accumulation is likely such as severed fingers and ears. The anticoagulant of leeches is also a fertile ground of research for surgeries in which an incision must be kept open. In addition, leech saliva is found to contain powerful antibiotics and anaesthetics which no doubt will prove useful in future medicinal practice (Sawyer, 1986).

Economic Importance for Humans: Negative

The medicinal leech is parasitic on humans and is a source of unpleasant emotions for leech victims and bystanders alike (Grzinke, 1974).

Conservation Status

The medicinal leech is rare throughout its range in Europe and extinct in much of its former range. This is due primarily to the overharvesting of leeches in the past century for medicinal use. Other factors contributing to the leech's reduced status is the alteration of its usual habitat and possibly a decrease in the frog population. Frogs are essential for leech development as its young cannot yet feed off mammals for its first two meals (Sawyer, 1986).

Other Comments

H. medicinalis has stimulated human imagination for centuries. Its intimate contact with humans have provoked a somewhat symbiotic relationship in which the leech feeds off humans and humans use the leech for medicine, stories and imagery in popular culture. (Grzimek, 1974; Sawyer 1986).

Contributors

Kathy Silverstein (author), University of Michigan-Ann Arbor.

Glossary

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

References

Grzimek, Dr. H.C. Bernard. 1974. Grzinek's Animal Life Encyclopedia. Vol. 1. Van Nostrand Reinhold co., NY.

Sawyer, Roy T. 1986. Leech Biology and Behaviour. Vol 1-2. Clarendon Press, Oxford.