Leeches: Hirudinea

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LEECHES: Hirudinea



Most leeches are flattened from top to bottom and measure
0.196 to 0.787 inches (5 to 20 millimeters) in length. The longest species may reach 17.72 inches (450 millimeters). Leeches have eyespots on the head that are able to detect movement from contrasting patterns of light and shadow. The mouth is located underneath the head and is surrounded by a sucker. The sucker helps draw blood and other bodily fluids into the mouth. Like earthworms, leeches have a clitellum (KLAI-teh-lum), a specialized collarlike band behind the head. The clitellum is filled with special tissues that secrete a protective covering, or case, for the eggs. Leeches are not covered with stiff bristles nor do they have paddlelike flaps. To gain traction, leeches have suckers at the front and rear of their bodies. The tail sucker is used for swimming, getting around, or remaining attached to a host. Blood-feeding species have special pouches that allow them to increase their intake of fluid. They can expand up to six times their normal weight.


Leeches live on all continents except Antarctica.


Leeches are found in a wide variety of habitats. Those living in the ocean and estuaries (EHS-chew-air-eez), or wide areas where rivers join the sea, are found on rocks, plants, or attached to fish or other sea animals. Species living on land are found in moist habitats, such as rainforests or wet coastal forests. They are usually found clinging to plants or under rocks. Freshwater species live in swamps, ponds, streams, and rivers where they are live on wood, rocks, vegetation, or on other animals.


Leeches are all carnivores (KAR-nih-vorz) and feed on the flesh or fluids of other animals. Some species feed only on the blood of their prey. Many leeches are predators that ambush a wide variety of invertebrates (in-VER-teh-brehts), animals without backbones. Prey includes insects, earthworms and their relatives, beach hoppers, snails, and freshwater clams. Predatory leeches swallow their prey whole or pierce the bodies of their victims with their retractable, needlelike mouthparts, or proboscis (pruh-BAH-suhs). These mouthparts are then used like a soda straw to suck out bodily fluids.

Blood-feeding leeches attack fishes, turtles, crocodiles and their relatives, frogs, ducks, geese, other water birds, and mammals, including humans. Some leeches feed on only one kind of animal. Others will feed on anything. If larger prey is not available, these leeches will survive on worms, insects, and other invertebrates. Although some blood-sucking species use a proboscis, most use their jaws to pierce the skin of their victims. They have two or three razor-sharp jaws, each shaped like half of a circular saw blade. Two-jawed leeches leave a V-shaped mark. Those with three jaws leave a Y-shaped wound. These leeches have chemicals in their saliva that prevent the blood of mammals from clotting, either at the wound or inside their own bodies.


Many leeches swim through the water with snakelike motions. They release their grip with the tail sucker and push off from a rock or plant, before wriggling their body back and forth. Species living on land move along the ground like an inchworm, stretching and shortening their bodies by using the suckers on both ends.

Leeches must mate to reproduce. Mating occurs when a leech attaches a sperm packet in the body of its mate. Some species have corresponding male and female organs that allow the placement of sperm directly into their mate's female reproductive organs. Fertilization (FUR-teh-lih-ZAY-shun) occurs inside the female's body. As the cocoon passes over the female reproductive organs, the developing eggs, or embryos (EHM-bree-ohz), are deposited inside.

The cocoons are either left in the soil, or attached to the bodies of other animals. Young leeches resemble the adult when they hatch. Fish leeches attach their egg cases to the bodies of crustaceans (krus-TAY-shuns), or water-dwelling animals with soft bodies covered by hard shells, such as shrimp or lobster. When fishes eat infested crustaceans, the young leeches attach themselves inside the fishes' mouth cavity. Some leeches carry their eggs in a clear sack under their body. When the adult finds a host, like a turtle or frog, the young hatch and also attach themselves to the host.


Leeches were used in the 1700s and 1800s to treat all kinds of ailments, from headaches to being overweight. It was highly unlikely that any of these uses was successful. Today, leeches are used to treat tiny blood clots that form after surgery. The anti-clotting properties of their saliva may also be useful for treating heart disease, possibly even cancer. Aquatic leeches are used to measure environmental health because they are sensitive to heavy metal pollution and low oxygen content in freshwater habitats.


Doctors use farm-raised leeches for several kinds of medical treatments. They are used to remove pools of thickened blood under the skin grafts of burn patients. A hungry leech can restore the circulation in clogged veins by sucking out blood clots. They are especially useful for improving the circulation in reattached body parts, such as fingers and ears, by helping to restore blood flow through reconnected veins.


One species of leech, the European medicinal leech, is listed by the World Conservation Union (IUCN) as Lower Risk, or at risk of becoming threatened with extinction in the future. They are threatened due to habitat loss and over-collecting. Other species with limited distributions may also be threatened.


Physical characteristics: The North American medicinal leech grows up to 3.9 inches (100 millimeters) long and 0.39 inches (10 millimeters) wide. The upper surface of the body is olive with a row of orange spots down the middle. The underside is orange. An arch of ten eyes is arranged in five pairs. Each of the three jaws has very fine small teeth.

Geographic range: This species is found in Eastern North America, from southern Canada to the Carolinas and along the Mississippi River drainages.

Habitat: This leech is found in naturally occurring freshwater lakes, ponds, streams, and marshes. It is usually found at the water surface near shore when at rest.

Diet: The North American medicinal leech feeds mostly on amphibian and fish blood, but occasionally attacks mammals. Blood meals are stored in the body up to several months.

Behavior and reproduction: This species is an excellent swimmer. They can detect prey up to several yards (meters) away by following waves in the water.

Sperm is deposited directly into the female reproductive organs of the mate. Up to ten or more eggs are laid in the cocoon. The cocoons are deposited on land near the edge of a body of water.

North American medicinal leeches and people: The North American medicinal leech was used in creation mythology of the Osage tribe of Native Americans. It was not widely used for blood-letting. This species was also seen in the film Stand by Me.

Conservation status: The North American medicinal leech is not considered threatened or endangered. ∎


Physical characteristics: Possibly the largest freshwater leech, the giant Amazonian leech can grow up to 17.72 inches (450 millimeters) long and 3.93 inches (100 millimeters) wide. Adults are dark gray-brown. Younger individuals have a broken stripe down their backs and patches of color on every third body segment. They have only one pair of eyes.

Geographic range: They live from the mouth of the Amazon River, north to Venezuela and the Guianas.

Habitat: The giant Amazonian leech lives in coastal wetland marshes.

Diet: Young leeches feed on amphibians. Adults usually attack caimans, anacondas, capybaras, and domestic cattle.

Behavior and reproduction: This species is a good swimmer and is usually found under rocks or in debris in the water while they digest their meals and carry their cocoons.

Sperm packets are attached to the body of the mate. Eggs are kept in a sac underneath the parent's body until they hatch and are carried to their first blood meal by the parent.

Giant Amazonian leeches and people: A special chemical isolated from the saliva of the giant Amazonian leech is sometimes used as a medical treatment to break down blood clots in humans.

Conservation status: The giant Amazonian leech is not considered endangered or threatened. ∎



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Elliott, J. M., and P. A. Tullett. "The Status of the Medicinal Leech Hirudo medicinalis in Europe and Especially in the British Isles." Biological Conservation 29 (1984): 15-26.

Wells, S., and W. Coombes. "The Status of and Trade in the Medicinal Leech." Traffic Bulletin 8 (1987): 64-69.

Web sites:

All About Leeches Web Page.http://www.invertebrate.ws/leech/index.htm (accessed on December 26, 2004).

Class Hirudinea. Leeches.http://lakes.chebucto.org/ZOOBENTH/BENTHOS/xxvi.html (accessed on December 26, 2004).

Leeches. Biological Indicators of Watershed Health.http://www.epa.gov/bioindicators/html/leeches.html (accessed on December 26, 2004).

Lovable Leeches.http://www.accessexcellence.org/LC/SS/leechlove.html (accessed on December 24, 2004).


The Biology of Annelids. Beaufort, SC: BioMedia Associates, 2000.

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