Leech Biology: Feeding, Sensory Adaptations, and Reproduction

Leeches are segmented worms, close relatives of earthworms, and they are best known for drinking blood. Yet blood is only one way they eat: many leeches hunt small animals or suck out their soft tissue instead.¹ The traits that make a blood-feeding leech good at its job, a bite you barely feel and blood that keeps flowing afterward, are also why surgeons still use them today.

How a Leech Bite Works and Keeps Blood Flowing

The medicinal leech has three muscular jaws arranged around its mouth.² Each jaw carries a row of tiny teeth along its edge.³ Together the three jaws leave a Y-shaped cut, often described as a small triangle on the skin. Tiny openings between the teeth release saliva straight into the wound, and the size of the teeth helps decide how long the wound keeps bleeding.⁴

Most people feel little or nothing, because the saliva is a mix of many substances and is generally thought to include compounds that dull sensation. It also contains substances that widen nearby blood vessels, which helps blood keep flowing toward the wound.

The best-known ingredient in that saliva is hirudin, a substance that stops blood from clotting.⁵ It locks onto the body’s main clotting enzyme and blocks it from working.⁶ Before the drug heparin was discovered, hirudin was the only remedy for preventing clots.⁷

Because the saliva stays in the tissue, a leech bite often keeps oozing for hours after the leech drops off. This bleeding is normal. Gentle pressure and a clean bandage usually stop it, though you may need to change the bandage a few times.

If you want to remove a leech, slide a fingernail under the sucker at each end rather than yanking. Salt, fire or chemicals can make a leech let go, but they may also make it regurgitate into the wound. Wash the bite with soap and water afterward, and see a doctor if redness spreads or bleeding won’t stop.

Not All Leeches Drink Blood

Some leeches feed on blood, some suck fluids and soft tissue from their prey, and some are predators that swallow prey whole.⁸ Blood-feeding water leeches have been found with the DNA of fish, turtles and birds in their guts.⁹ Fluid-and-tissue feeders mainly eat worms, insects and other arthropods, and snails.¹⁰ Predatory leeches mostly eat insects, other arthropods and worms.¹¹ All of these groups descend from blood-feeding ancestors, so predatory leeches gave up blood-feeding several separate times.¹²

The mouth parts reflect the diet. Blood-feeders cut with jaws, while predators that swallow prey whole do not need them. So a leech clamped onto your leg in a lake is almost certainly a blood-feeder. Many pond leeches never bite people at all.

Leeches live in fresh water, river mouths and the sea, and some live on land.¹³¹⁴ Because a leech’s gut holds DNA from what it ate, scientists can use leeches to find out which animals live in an area.¹⁵

How Leeches Sense Their Next Meal

Medicinal leeches can feel tiny ripples on the water’s surface.¹⁶ Sensitive hair-like cells in rings along the body pick up the movement of water.¹⁷ Even seeing waves move works: visual ripples alone trigger the leech’s nerves.¹⁸ Some leeches also have simple cup-shaped eyes that sense light and direction rather than sharp images.¹⁹ A person wading into a pond can draw leeches partly this way.

Once it reaches a warm surface, the decision gets more particular. Warm water at body temperature makes a leech probe and briefly attach, but not feed.²⁰ Blood or plasma at room temperature triggers the full sequence of probing, attaching, biting and drinking, because the leech tastes blood through its lips.²¹ Just two ingredients, salt and arginine, a building block of protein, are enough to make it feed, though neither is enough on its own.²² Leeches whose taste-sensing lip area is removed cannot find or drink blood at all.²³

A Giant Meal and How a Leech Handles It

A medicinal leech can drink up to 10 times its own body weight in one meal.²⁴ It swells from a flat, slender shape into a plump, rounded body, and it lets go on its own once full.

A meal that size leaves a large load of water and salt to get rid of quickly. 17 pairs of kidney-like organs filter out the excess.²⁵ After a blood meal, its salt output rises about 80-fold.²⁶ Some other leeches handle this much less efficiently and cannot survive in salty water.²⁷

The rest of the meal is digested slowly, so a blood-feeder can go a long time before it needs to eat again. The medicinal leech’s gut holds only a few kinds of bacteria.²⁸ One main gut bacterium, Aeromonas veronii, lives there as a partner.²⁹ The leech’s immune cells eat up other bacteria, but this one dodges them.³⁰ Immune proteins in the swallowed blood stay active and help keep other bacteria down.³¹ This limited community helps keep the stored blood from spoiling during the wait.

The same bacteria matter in medicine. Aeromonas can infect a human wound after leech therapy, which is why patients treated with leeches are usually given antibiotics. Hospitals also use leeches raised under controlled conditions rather than wild ones.

How Leeches Mate and Lay Eggs

Every leech is a hermaphrodite, meaning it carries both male and female organs at the same time.³²³³ Even so, leeches rarely fertilize themselves, so they still need a partner.³⁴

In medicinal leeches, one leech’s male opening must line up with the partner’s female opening one segment back.³⁵ Leeches keep their bellies down, so lining up takes a twisting, body-scanning dance.³⁶ Lip taste sensors likely help confirm the partner is a leech and in position.³⁷ Mating is sometimes mutual but usually one-way.³⁸

In another group of leeches, one partner jabs a sperm packet into the other’s body wall anywhere.³⁹ Within about 20 minutes a channel forms and the sperm are pushed into the body and spread through it.⁴⁰ This can injure or even kill a small leech.⁴¹

A band of thickened segments produces a protein cocoon for the eggs.⁴²⁴³ The cocoon is a tube sealed at each end by glue-like plugs.⁴⁴ You may spot these as small, spongy capsules stuck to rocks, plants or damp soil.

Some small pond leeches carry 3 to 7 cocoons, each with 2 to 20 eggs, on their bellies.⁴⁵ The young cling to the parent and stay for about three more weeks after hatching.⁴⁶

Why Hospitals Still Use Leeches

After reconstructive surgery, moved tissue can fail when blood flows in but can’t drain out.⁴⁷ Doctors apply leeches to draw off the trapped blood.⁴⁸ Leeches are most often used on reattached fingers or ears and on skin flaps with blocked drainage.⁴⁹ Leech therapy can also help protect against clots and damage when blood flow returns.⁵⁰

A nurse or doctor places the leech on the swollen, congested area and leaves it to feed until it drops off by itself. The bite keeps oozing afterward, which continues the drainage. The care team checks the tissue’s color and swelling and reapplies leeches as needed. Each leech is used on one patient only and then destroyed.

In one group of plastic surgery patients, about 87% fully recovered after leech treatment.⁵¹ Leeches complement surgery rather than replace it, and some tissue still fails. Infection from the leech’s gut bacteria is the main concern, and repeated bleeding can lower blood counts.