Leeches

Various species in the subclass Hirudinea

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Photo of a leech
Jim Rathert
Family

Various families in the subclass Hirudinea (leeches) in the phylum Annelida (annelids, the segmented worms)

Description

Leeches are segmented worms with flattened bodies. They are usually black or brown, although some species are brightly colored. Many leeches have attractive speckled or striped patterns. They are amazingly flexible, able to expand and contract to the point where it’s difficult to measure them. Each body segment is subdivided into annuli, which resemble wrinkles and add to the creature’s flexibility. There is a sucker at each end of the elongated body, and the suckers are often used for locomotion: Leeches can move by “looping” like an inchworm. The head of a leech holds the mouth sucker, usually at the more tapered (narrower) end. About a quarter of the way down from the head is the clitellum, which becomes more noticeable during times of reproduction (earthworms have this same swollen-looking structure, used in reproduction).

Size

Length: usually ½ to 2½ inches, although some U.S. species can grow to 10 or even 20 inches.

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Photo of a leech and a water penny both clinging to the same rock.
Leech and Water Penny
Leeches, which are worms, and water pennies, which are beetles, both have segmented bodies.

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Photo of a leech on a turtle's hind leg
Leech On Turtle Leg
Some leeches specialize in certain kinds of hosts while others are generalists. This leech is hanging from a turtle's hind leg.
Habitat and conservation

Leeches are most abundant in warm, calm, shallow waters of ponds and lakes, especially in areas choked with vegetation and various debris. They usually avoid light. Some species hide under stones or under submerged logs; others live in the bottom muck; others “loop” among plants in gently moving waters of streams. Not all leeches are bloodsuckers. All native leeches have a place in our aquatic ecosystems.

Foods

Many leeches are predatory, surprising small aquatic animals and eating them whole. But many species are parasites, equipped with jaws and muscles that enable them to adhere to the bodies of their hosts and consume blood. Different leech species specialize in different hosts, including mammals, reptiles, amphibians, fish, and snails. They usually secrete an enzyme that prevents the host’s blood from clotting, as well as an anesthetic, which prevents the host from noticing the “attack.”

image of Leeches Distribution Map
Distribution in Missouri

Statewide.

Life cycle

Leeches, like their relatives the earthworms, are hermaphrodites, which means that each leech is both male and female: A mating pair of leeches fertilize each other, and both individuals produce eggs as a result. Also like earthworms, the clitellum structure secretes a ringlike mucous cocoon to protect the eggs.

Human connections

For centuries, doctors treated various ailments by reducing the volume of blood in the patient’s body, and one way to do this was by attaching leeches, which gently removed that blood. Today, surgeons use the anticoagulants of leeches to keep blood flowing in reattached body parts.

Ecosystem connections

Humans are naturally repulsed by squishy bloodsuckers, but parasitism is an ancient way of life for many animals on Earth, and there is much to say for it. Parasites, unlike predators, generally do not kill their hosts, allowing them to live another day.